fix(reviewer-bot): stop transition spam and repair freshness recovery

[PLeVasseur]

Summary

• make transition notices one-shot, remove the false reassignment message, and backfill existing transition notices so already-affected PRs stop spamming
• simplify the PR comment observer workflow to build deferred payloads inline instead of bootstrapping bot source at runtime
• repair reviewer freshness recovery by clearing transition markers on reviewer activity, filtering bot-authored comment gaps, and conservatively recovering from visible submitted reviews

Testing

• uv run pytest .github/reviewer-bot-tests/test_reviewer_bot.py
• uv run pytest .github/reviewer-bot-tests/test_main.py
• uv run ruff check --fix

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