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>

.agents/skills/lib_py/loop/watcher.py

@@ -47,15 +47,42 @@ BACKOFF_MAX_S = 16.0
 CALLER_SCOPE_VARS = ("HERDR_WORKSPACE_ID", "HERDR_PANE_ID", "HERDR_TAB_ID")
 
 
+def caller_server(environ=None):
+    """The server whose pane this process runs in, "" when not in a pane.
+
+    HERDR_SESSION when present; otherwise the shape of HERDR_SOCKET_PATH, which
+    herdr exports to every pane (HERDR_SESSION is absent in a default-server
+    pane on herdr 0.8.2 - the orchestrator's usual home, and where the #23 leak
+    comes from). Same convention as lib.sh's _herdr_caller_server:
+
+        .../.config/herdr/herdr.sock                 -> "default"
+        .../.config/herdr/sessions/<name>/herdr.sock -> <name>
+
+    Matched by shape, not by $HOME. Anything else is "" (not in a pane).
+    """
+    src = os.environ if environ is None else environ
+    sess = (src.get("HERDR_SESSION") or "").strip()
+    if sess:
+        return sess
+    sock = (src.get("HERDR_SOCKET_PATH") or "").strip()
+    parts = sock.split("/")
+    if len(parts) >= 4 and parts[-1] == "herdr.sock":
+        if parts[-3:-1] == [".config", "herdr"]:
+            return "default"
+        if len(parts) >= 5 and parts[-4:-2] == ["herdr", "sessions"] and parts[-5] == ".config":
+            return parts[-2]
+    return ""
+
+
 def scoped_env(session=None, environ=None):
     """The environment for a wait aimed at `session`.
 
     Unchanged unless this process runs inside a pane of another server: only
     then are the caller's own workspace/pane/tab ids meaningless for the call
-    and dropped. No pane (no HERDR_SESSION) and same-server keep every value.
+    and dropped. No pane and same-server keep every value.
     """
     src = os.environ if environ is None else environ
-    caller = (src.get("HERDR_SESSION") or "").strip()
+    caller = caller_server(src)
     target = (session or "").strip()
     # `default` is the same sentinel build_argv reads: no --session flag, so
     # herdr answers on the ambient server - the caller's own.