Include author replies in re-review context

[asavs]

Summary

• Fetch recent review-thread comments alongside the opening bot comment and carry non-bot replies into the prior-thread handle map.
• Render recent author replies under unresolved prior bot threads with count and byte caps, excluding bot-authored replies.
• Tighten the review prompt so repeated prior findings must accept or refute author evidence against the PR-head source.

Validation

• C:\Program Files\Git\bin\bash.exe scripts/reviewer/tests/run-fixtures.sh -> skipped because this Windows Git Bash lacks flock.
• Direct Git Bash smoke test for append_prior_bot_inline_threads author-reply rendering passed.
• C:\Program Files\Git\bin\bash.exe -lc 'bash -n scripts/reviewer/lib/github.sh scripts/reviewer/lib/prompt.sh scripts/reviewer/tests/fixtures/prompt.sh scripts/reviewer/tests/run-fixtures.sh' passed.

Caveats

• This PR is intentionally thread-scoped; it does not add top-level PR conversation comments as re-review context.