Honor explicit no-more-tools Stop boundaries

[BananaAccurate]

Summary

• preserve newly queued files while honoring /tailtest defer and explicit no-further-tools boundaries
• read only the bounded latest user message from Codex-owned session transcripts
• fail closed for assistant text, fenced examples, and external transcript paths
• document the one-turn defer contract and add seven regression scenarios

Verification

• python -m pytest -q tests/test_stop_hook.py — 27 passed on Windows
• python3 -m pytest -q — 407 passed under Ubuntu/WSL from the exact clean worktree
• candidate adds no Ruff findings relative to origin/main (both report the same 15 pre-existing findings)
• live installed-plugin canary observed file change → CANARY_SAFE → turn completion, with no subsequent tool call; adversarial filename remained data and no PWNED artifact was created

[BananaAccurate]

@avansaber This is ready for maintainer review and merge. The PR is scoped to five files and is currently mergeable. Verification: 407 tests passed on Ubuntu/WSL from the exact clean worktree; 27 focused Stop-hook tests passed on Windows; the live installed-plugin canary completed immediately after CANARY_SAFE with no later tool call, and the adversarial filename remained data. Maintainer edits are enabled. I left release versioning/tagging to the maintainer workflow documented in CONTRIBUTING.md.