Capture stderr from the dedicated WM server startup

start_own_server() (used by prepare-envs, run, dump-config, bootstrap)
sent the subprocess's stderr to DEVNULL, so a crash before the server's
own logger initializes left no trace anywhere. The resulting timeout
error told the caller to "check logs for errors" that never existed.

Capture stderr to a file instead (creating its parent dir, which
ensure_running gets for free but this path doesn't), and inline its
content into the raised error so it survives even when the disk that
held it is gone by the time anyone looks (e.g. a CI runner).

Claude-Session: https://claude.ai/code/session_01XaMRTq9Sk67d8x8zEm7wBJ

Author: Aksem

src/finecode/wm_server/wm_lifecycle.py

@@ -180,6 +180,11 @@ def start_own_server(
     # Write empty content so the server overwrites rather than appends.
     port_file.write_text("")
 
+    stderr_path = startup_stderr_log_path()
+    # Unlike `ensure_running`, nothing else guarantees this directory exists
+    # first (there `_startup_lock()` creates it as a side effect before the
+    # log file is opened) — on a first-ever start in this venv it is missing.
+    stderr_path.parent.mkdir(parents=True, exist_ok=True)
     logger.info(f"Starting dedicated FineCode WM server in {workdir}")
     command = [
         sys.executable,
@@ -196,13 +201,17 @@ def start_own_server(
 
     # No own session, unlike `ensure_running`: a dedicated server belongs to
     # exactly one client and is reachable only through that client's port file,
-    # so outliving it would leave a ghost nobody can find.
-    subprocess.Popen(
-        command,
-        cwd=str(workdir),
-        stdout=subprocess.DEVNULL,
-        stderr=subprocess.DEVNULL,
-    )
+    # so outliving it would leave a ghost nobody can find. stderr is still
+    # captured to a file (not DEVNULL'd) so a crash before the server's own
+    # logger is initialized is not silently lost — see `wait_until_ready_from_file`,
+    # whose error message points here.
+    with open(stderr_path, "w") as stderr_file:
+        subprocess.Popen(
+            command,
+            cwd=str(workdir),
+            stdout=subprocess.DEVNULL,
+            stderr=stderr_file,
+        )
     return port_file
 
 
@@ -289,7 +298,19 @@ async def wait_until_ready_from_file(
         except (FileNotFoundError, ValueError, OSError):
             pass
         await asyncio.sleep(0.5)
+    stderr_path = startup_stderr_log_path()
+    # Inlined, not just referenced by path: on CI the runner's disk is gone by
+    # the time anyone could go look, so the message itself is the only place
+    # this content is ever seen.
+    try:
+        stderr_tail = stderr_path.read_text().strip()
+    except OSError:
+        stderr_tail = ""
+    detail = (
+        f"Captured stderr ({stderr_path}):\n{stderr_tail}"
+        if stderr_tail
+        else f"{stderr_path} is empty — the process is still starting, not crashing."
+    )
     raise TimeoutError(
-        f"Dedicated FineCode WM server did not start within {timeout}s. "
-        "Check logs for errors."
+        f"Dedicated FineCode WM server did not start within {timeout}s.\n{detail}"
     )