fix: prompt agents to read repo instructions

[pilipilisbot]

Summary

• add a packaged repository-instruction prompt rule for files such as AGENTS.md, CLAUDE.md, and similar local guidance
• keep those files subordinate to bridge policy, work intent, prompt-injection, and tool-safety rules
• allow operators to override the new prompt rule through policy.json promptOverrides.rules.repo_instructions

Tests

• .venv/bin/python -m pytest tests/test_prompt_rules.py tests/test_policy.py -q
• .venv/bin/python -m pytest -q
• git diff --check

Closes #43

[pilipilisbot]

Post-merge cleanup for PR #44 is complete.

Summary:

• merged into main as 8331012c170564cc03becd6f3be849f428d28c75;
• post-merge workflows passed: tests 26400242145 and release 26400242119;
• release v0.18.5 was published: https://github.com/pilipilisbot/github-agent-bridge/releases/tag/v0.18.5
• the canonical checkout is clean on main at v0.18.5 (245f6de);
• there was no dedicated PR worktree left to remove, and the handle-repo-instruction-files branch is gone locally/remotely.

No further cleanup is needed for this PR.