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/.pi-map.index.md

@@ -2,14 +2,23 @@
 dir: apps/api
 
 ## role
-FastAPI-based backend API service for managing projects, git repositories, and development tools in a self-hosted development platform.
+FastAPI backend API that manages projects, git repositories, development tools, and tool instances via Docker Compose
 ## parent
 index: apps/.pi-map.index.md
 map: apps/.pi-map.md
 ## children
+- apps/api/.mypy_cache
+  index: apps/api/.mypy_cache/.pi-map.index.md
+  map: apps/api/.mypy_cache/.pi-map.md
 - apps/api/.pi-lens
   index: apps/api/.pi-lens/.pi-map.index.md
   map: apps/api/.pi-lens/.pi-map.md
+- apps/api/.pytest_cache
+  index: apps/api/.pytest_cache/.pi-map.index.md
+  map: apps/api/.pytest_cache/.pi-map.md
+- apps/api/.ruff_cache
+  index: apps/api/.ruff_cache/.pi-map.index.md
+  map: apps/api/.ruff_cache/.pi-map.md
 - apps/api/alembic
   index: apps/api/alembic/.pi-map.index.md
   map: apps/api/alembic/.pi-map.md