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/tests/unit/test_terminal_session.py

@@ -0,0 +1,64 @@
+"""Unit tests for TerminalSession docker exec invocation."""
+
+import uuid
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from src.services.terminal.terminal_session import TerminalSession
+
+
+@pytest.mark.unit
+@pytest.mark.asyncio
+async def test_start_passes_container_user_to_docker_exec() -> None:
+    """When container_user is set, docker exec receives --user <user>."""
+    session = TerminalSession(
+        session_id=str(uuid.uuid4()),
+        instance_id=uuid.uuid4(),
+        container_id="container-123",
+        container_user="dev",
+    )
+
+    with patch(
+        "src.services.terminal.terminal_session.pty.openpty",
+        return_value=(1, 2),
+    ):
+        with patch(
+            "src.services.terminal.terminal_session.asyncio.create_subprocess_exec",
+            new=AsyncMock(),
+        ) as mock_exec:
+            with patch("src.services.terminal.terminal_session.os.close"):
+                await session.start()
+
+    args, _kwargs = mock_exec.call_args
+    assert "docker" in args
+    assert "exec" in args
+    assert "--user" in args
+    user_index = args.index("--user")
+    assert args[user_index + 1] == "dev"
+    assert "container-123" in args
+
+
+@pytest.mark.unit
+@pytest.mark.asyncio
+async def test_start_omits_user_when_not_configured() -> None:
+    """Without container_user, docker exec does not receive --user."""
+    session = TerminalSession(
+        session_id=str(uuid.uuid4()),
+        instance_id=uuid.uuid4(),
+        container_id="container-123",
+    )
+
+    with patch(
+        "src.services.terminal.terminal_session.pty.openpty",
+        return_value=(1, 2),
+    ):
+        with patch(
+            "src.services.terminal.terminal_session.asyncio.create_subprocess_exec",
+            new=AsyncMock(),
+        ) as mock_exec:
+            with patch("src.services.terminal.terminal_session.os.close"):
+                await session.start()
+
+    args, _kwargs = mock_exec.call_args
+    assert "--user" not in args