Summary

• feat: Tool schema diagnostics & duplicate name detection ([Feature]: Add lightweight validation for tool_info schemas before tool registration #75)
• feat: New PolicyHook with phase restrictions, fail-closed, and sync/async timeout
• feat: on_handoff support with blocking, trace propagation, and source agent lifecycle closure
• docs: Updated tools & hooks documentation, roadmap, version to 0.9.4
• test: Three new test modules covering new features
• ci: Expanded the Python 3.10/3.11/3.12 matrix to run the complete test suite instead of three focused modules

Testing

• Clean-worktree full test suite: 100 passed in 7.88s
• GitHub CI: 100 passed on Python 3.10, 3.11, and 3.12
• CI command: PYTHONPATH=. python -m pytest -q
• compileall and git diff --check pass

Notes

• No new News entry for v0.9.4, only corrected v0.9.3 status to "released" in English news.
• No CCS or other external policy dependency was added.
• pytest-cov and coverage thresholds remain follow-up work.

Closes #75.
Related to #76.