fix: muxterm binary discovery — find local build before PATH search

_find_muxterm_binary() now resolves in this order:
  1. Explicit binary_path argument (unchanged)
  2. MUXTERM_BINARY env var (new)
  3. <repo_root>/muxterm/muxterm — the go build output for dev installs (new)
  4. shutil.which('muxterm') — PATH fallback (unchanged)

Before this fix the service logged 'muxterm binary not found; terminal
feature disabled' on every restart because the binary is not on PATH
and no explicit path was passed from main.py.

Also fix stop_muxterm() to catch ProcessLookupError from terminate()
when the process has already exited (was only caught in the wait_for
block, not the terminate() call itself), which caused a noisy
'Application shutdown failed' traceback in the service journal.

muxplex/muxterm.py

@@ -16,6 +16,8 @@ Internal:
 
 import asyncio
 import logging
+import os
+import pathlib
 import shutil
 
 logger = logging.getLogger(__name__)
@@ -35,16 +37,37 @@ _restart_task: asyncio.Task | None = None  # type: ignore[type-arg]
 def _find_muxterm_binary(binary_path: str | None = None) -> str:
     """Locate the muxterm binary.
 
-    Checks explicit *binary_path* first, then falls back to
-    ``shutil.which('muxterm')``.  Raises :exc:`FileNotFoundError` if
-    the binary cannot be found.
+    Resolution order:
+      1. Explicit *binary_path* argument.
+      2. ``MUXTERM_BINARY`` environment variable.
+      3. ``<repo_root>/muxterm/muxterm`` — the build output for dev/local installs
+         (package lives at ``<repo_root>/muxplex/``, binary at
+         ``<repo_root>/muxterm/muxterm``).
+      4. ``shutil.which('muxterm')`` — installed on PATH.
+
+    Raises :exc:`FileNotFoundError` if none of the above resolves.
     """
     if binary_path is not None:
         return binary_path
+
+    env_path = os.environ.get("MUXTERM_BINARY")
+    if env_path:
+        candidate = pathlib.Path(env_path)
+        if candidate.is_file() and os.access(candidate, os.X_OK):
+            return str(candidate)
+
+    # Dev/local-install: binary is built next to the Python package directory.
+    # muxterm.py lives in  <repo_root>/muxplex/
+    # muxterm binary is at <repo_root>/muxterm/muxterm
+    local_candidate = pathlib.Path(__file__).parent.parent / "muxterm" / "muxterm"
+    if local_candidate.is_file() and os.access(local_candidate, os.X_OK):
+        return str(local_candidate)
+
     found = shutil.which("muxterm")
     if found is None:
         raise FileNotFoundError(
-            "muxterm binary not found on PATH; pass binary_path explicitly"
+            "muxterm binary not found; tried MUXTERM_BINARY env var, "
+            f"{local_candidate}, and PATH. Build it with `cd muxterm && go build .`"
         )
     return found
 
@@ -157,9 +180,15 @@ async def stop_muxterm() -> None:
         _restart_task = None
 
     if _muxterm_process is not None:
-        _muxterm_process.terminate()
+        try:
+            _muxterm_process.terminate()
+        except ProcessLookupError:
+            pass  # already exited
         try:
             await asyncio.wait_for(_muxterm_process.wait(), timeout=5.0)
         except (asyncio.TimeoutError, ProcessLookupError):
-            _muxterm_process.kill()
+            try:
+                _muxterm_process.kill()
+            except ProcessLookupError:
+                pass
         _muxterm_process = None

muxplex/tests/test_muxterm.py

@@ -46,16 +46,24 @@ def test_find_muxterm_binary_explicit_path(tmp_path):
 
 
 def test_find_muxterm_binary_uses_which():
-    """Falls back to shutil.which('muxterm') when no explicit path."""
-    with patch("shutil.which", return_value="/usr/bin/muxterm") as mock_which:
+    """Falls back to shutil.which('muxterm') when no env var and no local binary."""
+    with (
+        patch.dict("os.environ", {"MUXTERM_BINARY": ""}),  # suppress env override
+        patch("muxplex.muxterm.pathlib.Path.is_file", return_value=False),  # no local build
+        patch("shutil.which", return_value="/usr/bin/muxterm") as mock_which,
+    ):
         result = _find_muxterm_binary()
         mock_which.assert_called_once_with("muxterm")
         assert result == "/usr/bin/muxterm"
 
 
 def test_find_muxterm_binary_raises_when_not_found():
-    """Raises FileNotFoundError if muxterm is not found."""
-    with patch("shutil.which", return_value=None):
+    """Raises FileNotFoundError if muxterm is not found anywhere."""
+    with (
+        patch.dict("os.environ", {"MUXTERM_BINARY": ""}),  # suppress env override
+        patch("muxplex.muxterm.pathlib.Path.is_file", return_value=False),  # no local build
+        patch("shutil.which", return_value=None),
+    ):
         with pytest.raises(FileNotFoundError):
             _find_muxterm_binary()