feat(pr-review): let rework agent skip non-actionable bot comments via skip file

[chihsuan]

Context

ignored_reviewers (PR #122) silences whole bot accounts, but a review bot can post both actionable and non-actionable comments. The rework agent should be able to skip a single clearly non-actionable bot comment without silencing the account.

TL;DR

Let the rework agent mark individual non-actionable bot comments to skip, so they get no auto-reply — humans are never skippable.

Summary

• Prompt: agent writes non-actionable bot comment ids to .symphony-skip-comments.json instead of replying (path d); never for human reviewers.
• Poller: consumes the skip file (reads + deletes), suppresses the auto-reply for those ids, guarded to bot authors only and intersected with the current batch; logs what it skipped.
• Workspace: adds the skip file to the worktree git exclude so it is never tracked or committed.
• Local workspaces only; missing/malformed file degrades to normal reply behavior.

Alternatives

• Turning off reply_after_addressing so the agent owns all replies: rejected, loses the actionable-reply safety net.
• Only ignored_reviewers: rejected, it is per-account and cannot skip one comment from an otherwise-actionable bot.

Test Plan

• make all
• Poller: bot id in skip file gets no reply; human id ignored (guard); skip file consumed.
• Workspace: skip file is git-excluded in a created worktree.
• mix format --check-formatted, mix specs.check, mix credo on changed files.