fix: run tool terminal sessions as container user instead of root

- Remove compose-level user: 0:0 override from manifest_compiler.py so the
  entrypoint can start as root, fix mount ownership, and drop privileges to
  the container user internally.
- Add get_manifest_container_user() helper to resolve the manifest-declared
  container user (with uid:gid fallback).
- Pass container user through TerminalSession, TerminalManager, and the
  terminal WebSocket handler so docker exec is invoked with --user <user>.
- Update and add unit tests for the manifest compiler and terminal session.
- Record the additional root-user fix in the fix-pi-container-mount-permissions
  OpenSpec change/tasks.

Quality gates: pytest tests/unit/ (226 passed), pytest tests/services/test_terminal_manager_multi.py (7 passed), ruff check on changed files (clean), mypy on changed files (clean)

apps/api/src/services/terminal/terminal_manager.py

@@ -72,9 +72,10 @@ class TerminalManager:
                 session_id,
                 instance_id,
             )
-            session = self._sessions.pop(key, None)
-            if session:
-                await session.close()
+            idle_session = self._sessions.get(key)
+            if idle_session is not None:
+                del self._sessions[key]
+                await idle_session.close()
                 # Update DB status fire-and-forget
                 asyncio.create_task(self._mark_closed_in_db(session_id))
 
@@ -141,6 +142,7 @@ class TerminalManager:
         startup_command: str | None = None,
         name: str | None = None,
         session_id: str | None = None,
+        container_user: str | None = None,
     ) -> TerminalSession:
         """Create a new terminal session for an instance.
 
@@ -152,6 +154,8 @@ class TerminalManager:
             container_id: Docker container ID.
             startup_command: Optional startup command to run.
             name: Optional session name (auto-generated if omitted).
+            session_id: Optional explicit session UUID.
+            container_user: Optional container user for docker exec.
 
         Returns:
             The newly created TerminalSession.
@@ -177,6 +181,7 @@ class TerminalManager:
             container_id=container_id,
             startup_command=startup_command,
             name=name,
+            container_user=container_user,
         )
         await session.start(startup_command=startup_command)
 
@@ -201,6 +206,7 @@ class TerminalManager:
         instance_id: uuid.UUID,
         container_id: str,
         startup_command: str | None = None,
+        container_user: str | None = None,
     ) -> TerminalSession:
         """Get existing session or create a new one.
 
@@ -243,6 +249,7 @@ class TerminalManager:
             container_id=container_id,
             startup_command=startup_command,
             name="Session 1",
+            container_user=container_user,
         )
         await session.start(startup_command=startup_command)
         self._sessions[key] = session
@@ -358,6 +365,7 @@ class TerminalManager:
         startup_command: str | None = None,
         session_id: str | None = None,
         name: str | None = None,
+        container_user: str | None = None,
     ) -> TerminalSession:
         """Reset a session by killing it and creating a new one.
 
@@ -367,6 +375,7 @@ class TerminalManager:
             startup_command: Optional startup command.
             session_id: Specific session to reset. If None, resets the default session.
             name: Optional name to preserve for the new session.
+            container_user: Optional container user for docker exec.
 
         Returns:
             The newly created TerminalSession.
@@ -400,6 +409,7 @@ class TerminalManager:
             container_id=container_id,
             startup_command=startup_command,
             name=old_name or ("Session 1" if target_session_id == "default" else None),
+            container_user=container_user,
         )
         await new_session.start(startup_command=startup_command)
         self._sessions[key] = new_session