fix(herdr): prevent pane misattribution and prioritize idle-pane reuse in new-session

- Restrict generic agent_kind fallback in _resolve_herdr_pane_id to bare-kind searches
- Exclude 'agent' key from exact-match loop to enforce R-1 fail-closed contract
- Prioritize reusing existing idle pane on new-session to prevent unbounded pane proliferation
- Wrap final_cmd with env prefix to preserve env_flags when reusing idle pane

.agents/skills/lib.sh

@@ -29,7 +29,7 @@ export WORKSPACE_ROOT
 # Framework semantic version. Single runtime source of truth; kept in lockstep
 # with VERSIONS.md and the 8 SKILL.md frontmatters by tests/test_version_consistency.py.
 # NOTE: unlike other MAM_* variables this one is intentionally NOT env-overridable.
-MAM_VERSION="4.1.1"
+MAM_VERSION="4.1.2"
 export MAM_VERSION
 AGENT_SESSIONS_YAML="${AGENT_SESSIONS_YAML:-$WORKSPACE_ROOT/.mam/agent-sessions.yaml}"
 
@@ -412,7 +412,7 @@ try:
     if tws:
         panes = [p for p in panes if p.get('workspace_id') == tws]
     # ISSUE-2: exact match first, then agent-sessions.yaml lookup fallback
-    for key in ('label', 'name', 'agent'):
+    for key in ('label', 'name'):
         for p in panes:
             v = p.get(key)
             if v and (v == tn or v == tsa):
@@ -433,7 +433,13 @@ try:
                     break
         except Exception as e:
             sys.stderr.write(f'warning: yaml load failed: {e}\n')
-    if agent_kind:
+    # BUG-1: only trust the generic agent_kind fallback when the caller's
+    # own target string IS the bare kind (e.g. target == 'agy'). Using a
+    # *derived* kind (looked up via the yaml entry matched by tn/tsa) to
+    # then broadly match any pane of this kind is unsound whenever another,
+    # unrelated same-kind session is alive in the workspace - it misattributes
+    # that unrelated pane as this target even though their names differ.
+    if agent_kind and (tn == agent_kind or tsa == agent_kind):
         matching = [p.get('pane_id') for p in panes if p.get('agent') == agent_kind and p.get('pane_id')]
         if len(matching) == 1:
             print(matching[0])
@@ -702,6 +708,32 @@ except Exception:
     ws_id=""
     target_pane=""
     if [ -n "$existing_ws" ]; then
+      # BUG-4: prefer an already-idle (agent-less) pane in the existing
+      # workspace over splitting a new one — avoids unbounded pane
+      # proliferation and 2x2 grid-layout churn when a prior agent's pane
+      # was left behind idle.
+      idle_pane=$(_real_herdr pane list 2>/dev/null | TARGET_WS="$existing_ws" python3 -c "
+import sys, json, os
+target_ws = os.environ.get('TARGET_WS', '')
+try:
+    d = json.loads(sys.stdin.read())
+    panes = d.get('result', {}).get('panes', [])
+    for p in panes:
+        if p.get('workspace_id') == target_ws and not p.get('agent') and p.get('pane_id'):
+            print(p.get('pane_id'))
+            break
+except Exception:
+    pass
+" 2>/dev/null || echo "")
+
+      if [ -n "$idle_pane" ]; then
+        target_pane="$idle_pane"
+        if [ -n "$final_cmd" ]; then
+          idle_env="${env_flags//--env /}"
+          idle_env="${idle_env:+$idle_env }HERDR_WORKSPACE_ID=$existing_ws"
+          final_cmd="env $idle_env $final_cmd"
+        fi
+      else
       # W2a: Determine split direction policy via pane layout
       sample_pane=$(_real_herdr pane list 2>/dev/null | TARGET_WS="$existing_ws" python3 -c "
 import sys, json, os
@@ -751,6 +783,7 @@ except Exception:
     pass
 " 2>/dev/null || echo "")
       fi
+      fi
     fi
 
     if [ -z "$existing_ws" ] || [ -z "$target_pane" ]; then