Summary

• pass the ACP session cwd into AIAgent so project context discovery uses the same workspace as ACP tool execution
• invalidate the cached system prompt when an ACP session changes cwd so context files are rebuilt from the new workspace
• initialize ACP subdirectory hint tracking from the explicit session cwd and add regression coverage for both the agent and session manager paths

Closes #11515

Testing

• source venv/bin/activate && python -m pytest tests/run_agent/test_run_agent.py -q -k 'TestBuildSystemPrompt'
• source venv/bin/activate && python -m pytest tests/acp/test_session.py -q
• source venv/bin/activate && python -m pytest tests/ -q

Full suite note

The full suite still fails in this environment on existing unrelated tests and missing optional deps (acp, fastapi), plus current gateway/Discord/transcription failures. The ACP and run_agent coverage for this change passed locally.