fix(issue-23): the caller's server comes from HERDR_SOCKET_PATH, not only HERDR_SESSION

The live trial of this branch found the cross-server verdict inert on this
host: _herdr_call_is_cross_server read the caller's server from HERDR_SESSION,
and herdr 0.8.2 does not export that variable to a pane of the default server
- the orchestrator's usual home, and exactly the pane the #23 leak comes from.
Every call was judged "not crossing", the three ids were never dropped, and
all four trial onboardings ended "no started within 180s". My own pane sits in
a named server and does carry HERDR_SESSION, which is how the assumption got
past me.

HERDR_SOCKET_PATH is exported to every pane and its layout names the server:
.../.config/herdr/herdr.sock is the default server and
.../.config/herdr/sessions/<name>/herdr.sock a named one - the convention
`herdr session list --json` reports and the shim already uses to find a named
server's socket. _herdr_caller_server prefers HERDR_SESSION when a herdr does
export it, otherwise reads the socket's shape (suffix-matched, not $HOME-
anchored), and yields "" for anything unrecognised so an exported scope is
respected rather than guessed away. The rest of the rule is unchanged from
3bcaa39: a `default` or empty target is the ambient server, no caller server
means no pane, caller == target is the same server. watcher.caller_server
applies the same rule for the loop's own herdr calls.

Reproduced against the live server in the trial's exact env: 10a678c answers
"NOT crossing / has-session NOT FOUND"; this commit answers "caller server
default / crossing / FOUND". S1c pins that env end to end with the recorder
asserting the ids never reach herdr; S1d-S1h pin the verdict, the named-socket
same-server and cross-server cases, HERDR_SESSION precedence, and the
unrecognised layout. The 3bcaa39 tests are unchanged and still pass.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/test_issue23_24_startup_path.py

@@ -35,7 +35,7 @@ def _run_lib(tmp_path, body, env=None):
     script = 'source "%s"\n_init_herdr_isolation\n%s\n' % (lib_path, body)
     run_env = dict(os.environ)
     for var in ("HERDR_SESSION", "HERDR_SESSION_NAME", "HERDR_WORKSPACE_ID",
-                "HERDR_PANE_ID", "HERDR_TAB_ID"):
+                "HERDR_PANE_ID", "HERDR_TAB_ID", "HERDR_SOCKET_PATH"):
         run_env.pop(var, None)
     run_env.update(env or {})
     return subprocess.run(["bash", "-c", script], capture_output=True,
@@ -110,6 +110,16 @@ CROSS = {"HERDR_SESSION": "default", "HERDR_SESSION_NAME": "mam-isolated",
          "HERDR_WORKSPACE_ID": "w5", "HERDR_PANE_ID": "w5:p2",
          "HERDR_TAB_ID": "w5:t1"}
 
+# The orchestrator pane as the live trial of this branch measured it (herdr
+# 0.8.2): no HERDR_SESSION at all, the default server's socket, and that
+# server's own ids. This is the env in which the HERDR_SESSION-based verdict
+# was always "not crossing" and #23 reproduced unchanged.
+DEFAULT_SOCK = "/home/someone/.config/herdr/herdr.sock"
+NAMED_SOCK = "/home/someone/.config/herdr/sessions/%s/herdr.sock"
+TRIAL_PANE = {"HERDR_SOCKET_PATH": DEFAULT_SOCK, "HERDR_WORKSPACE_ID": "w6",
+              "HERDR_PANE_ID": "w6:p1", "HERDR_TAB_ID": "w6:t1",
+              "HERDR_SESSION_NAME": "multi-agent-mux"}
+
 
 def test_s1_cross_server_call_drops_the_caller_panes_scope_ids(
         mam_sandbox, mock_herdr, mock_agents, monkeypatch):
@@ -190,6 +200,83 @@ def test_s2b_a_caller_outside_any_pane_keeps_the_explicit_scope(
     assert res.returncode == 0, res.stdout + res.stderr
 
 
+def test_s1c_the_real_default_server_pane_is_a_crossing(
+        mam_sandbox, mock_herdr, mock_agents, monkeypatch):
+    """S1c (trial finding, req. 4): the caller's server comes from the socket
+    path when HERDR_SESSION is absent. In the measured orchestrator pane the
+    three ids must be dropped and the agent on the named server found."""
+    tmp_path = mam_sandbox
+    env_log = _install_env_recorder(tmp_path, monkeypatch)
+    _seed_agent(mock_herdr, tmp_path, "trial-agy", "w1", "w1:p5")
+
+    res = _run_lib(tmp_path, 'herdr has-session -t trial-agy', env=TRIAL_PANE)
+    assert res.returncode == 0, "the trial's exact failure: " + res.stdout + res.stderr
+
+    calls = _recorded(env_log)
+    assert calls, "the recorder saw no herdr call"
+    for call in calls:
+        assert call["HERDR_WORKSPACE_ID"] is None, call
+        assert call["HERDR_PANE_ID"] is None, call
+        assert call["HERDR_TAB_ID"] is None, call
+
+
+def test_s1d_the_verdict_itself_on_the_trial_pane(mam_sandbox):
+    """S1d: the function the trial printed, in the trial's env: crossing."""
+    res = _run_lib(mam_sandbox,
+                   'echo "caller=$(_herdr_caller_server)"\n'
+                   'if _herdr_call_is_cross_server; then echo VERDICT=crossing; '
+                   'else echo VERDICT=not-crossing; fi',
+                   env=TRIAL_PANE)
+    assert "caller=default" in res.stdout, res.stdout + res.stderr
+    assert "VERDICT=crossing" in res.stdout, res.stdout + res.stderr
+
+
+def test_s1e_a_named_server_pane_calling_its_own_server_keeps_its_scope(
+        mam_sandbox, mock_herdr, mock_agents):
+    """S1e (req. 4): a caller in a named server's pane (socket under
+    sessions/<name>/) targeting that same server is not a crossing - the
+    exported workspace id still scopes, as in S2."""
+    tmp_path = mam_sandbox
+    _seed_agent(mock_herdr, tmp_path, "creator-agy-01", "w1", "w1:p5")
+    same = {"HERDR_SOCKET_PATH": NAMED_SOCK % "mam-isolated",
+            "HERDR_SESSION_NAME": "mam-isolated", "HERDR_WORKSPACE_ID": "w2"}
+    res = _run_lib(tmp_path, 'echo "caller=$(_herdr_caller_server)"\nherdr has-session -t creator-agy-01',
+                   env=same)
+    assert "caller=mam-isolated" in res.stdout, res.stdout + res.stderr
+    assert res.returncode == 1, "same server: the explicit w2 scope must still hide a w1 agent"
+
+
+def test_s1f_a_named_server_pane_calling_another_named_server_is_a_crossing(
+        mam_sandbox, mock_herdr, mock_agents):
+    """S1f: named socket, different named target, no HERDR_SESSION - crossing."""
+    tmp_path = mam_sandbox
+    _seed_agent(mock_herdr, tmp_path, "creator-agy-01", "w1", "w1:p5")
+    other = {"HERDR_SOCKET_PATH": NAMED_SOCK % "mam-other",
+             "HERDR_SESSION_NAME": "mam-isolated", "HERDR_WORKSPACE_ID": "w2"}
+    res = _run_lib(tmp_path, 'herdr has-session -t creator-agy-01', env=other)
+    assert res.returncode == 0, res.stdout + res.stderr
+
+
+def test_s1g_herdr_session_wins_over_the_socket_when_both_are_present(mam_sandbox):
+    """S1g (req. 1): HERDR_SESSION, when a herdr exports it, is authoritative
+    over the socket-derived name."""
+    res = _run_lib(mam_sandbox, 'echo "caller=$(_herdr_caller_server)"',
+                   env={"HERDR_SESSION": "from-herdr", "HERDR_SOCKET_PATH": DEFAULT_SOCK})
+    assert "caller=from-herdr" in res.stdout, res.stdout + res.stderr
+
+
+def test_s1h_an_unrecognised_socket_layout_is_not_in_a_pane(mam_sandbox, mock_herdr, mock_agents):
+    """S1h: a socket path that is neither layout yields no caller server, so
+    an exported scope is respected (today's behaviour), never guessed away."""
+    tmp_path = mam_sandbox
+    _seed_agent(mock_herdr, tmp_path, "creator-agy-01", "w1", "w1:p5")
+    res = _run_lib(tmp_path, 'echo "caller=[$(_herdr_caller_server)]"\nherdr has-session -t creator-agy-01',
+                   env={"HERDR_SOCKET_PATH": "/tmp/odd/herdr.sock",
+                        "HERDR_SESSION_NAME": "mam-isolated", "HERDR_WORKSPACE_ID": "w2"})
+    assert "caller=[]" in res.stdout, res.stdout + res.stderr
+    assert res.returncode == 1, res.stdout + res.stderr
+
+
 def test_s3_shim_contract_regressions_still_pass(mam_sandbox):
     """S3: H-18 / H-21 / H-23 are the tests that fix today's scoping meaning.
     Run them explicitly as this change's regression guard."""
@@ -291,6 +378,18 @@ def test_s13_the_loop_watcher_applies_the_same_rule(mam_sandbox):
     none_target = watcher.scoped_env(None, environ=caller_pane)
     assert none_target["HERDR_WORKSPACE_ID"] == "w5"
 
+    # trial finding: no HERDR_SESSION, the caller's server comes from the socket
+    trial = {"HERDR_SOCKET_PATH": DEFAULT_SOCK, "HERDR_WORKSPACE_ID": "w6",
+             "HERDR_PANE_ID": "w6:p1", "HERDR_TAB_ID": "w6:t1"}
+    assert watcher.caller_server(trial) == "default"
+    assert "HERDR_WORKSPACE_ID" not in watcher.scoped_env("multi-agent-mux", environ=trial)
+    named_same = {"HERDR_SOCKET_PATH": NAMED_SOCK % "iso-x", "HERDR_WORKSPACE_ID": "w2"}
+    assert watcher.caller_server(named_same) == "iso-x"
+    assert watcher.scoped_env("iso-x", environ=named_same)["HERDR_WORKSPACE_ID"] == "w2"
+    assert "HERDR_WORKSPACE_ID" not in watcher.scoped_env("iso-y", environ=named_same)
+    assert watcher.caller_server({"HERDR_SOCKET_PATH": "/tmp/odd/herdr.sock"}) == ""
+    assert watcher.caller_server({"HERDR_SESSION": "s", "HERDR_SOCKET_PATH": DEFAULT_SOCK}) == "s"
+
     seen = {}
 
     def _run(argv, **kw):