Reply to GitHub review comments when implementing fixes

[Hrovatin]

Summary

When mach6-implement fixes review findings, the results are only reported in the terminal and via a top-level <!-- mach6-progress --> comment (posted by mach6-push). The individual review thread comments that originated the findings are never replied to on GitHub, making it hard for reviewers to see which findings were addressed without reading a monolithic progress comment.

Current Behavior

1. mach6-review posts review findings as a top-level PR comment (<!-- mach6-review -->)
2. mach6-implement <pr> <finding-numbers> fixes the specified findings locally
3. mach6-push posts a top-level <!-- mach6-progress --> comment summarizing committed changes
4. Individual review comments/threads receive no replies — reviewers must cross-reference the progress comment to understand what was fixed

Proposed Behavior

When mach6-implement fixes review findings, it should reply to the corresponding review comments on GitHub indicating what was fixed. This could happen either:

• During implementation (in mach6-implement itself), or
• During push (in mach6-push), alongside the existing progress comment

Each reply should reference what was changed and how the finding was addressed.

Acceptance Criteria

• After implementing fixes for review findings, each addressed finding's review comment receives a reply on GitHub
• Replies indicate what was fixed and how (e.g. file changed, approach taken)
• The existing top-level <!-- mach6-progress --> comment from mach6-push continues to work as before
• Works with the current gh CLI tooling (e.g. gh api for thread replies since gh pr comment only creates top-level comments)

Technical Notes

• All current GitHub interaction uses top-level gh pr comment — there is no existing code for replying to review threads
• GitHub API for replying to review comments: POST /repos/{owner}/{repo}/pulls/{pull_number}/comments/{comment_id}/replies
• The mach6-review findings are posted as a single top-level comment, not as individual inline review comments — so the "reply" target would need to be the review comment body or a new mechanism to correlate findings to threads
• Review subagents are constrained to never post to GitHub — the orchestrating skill handles all GitHub interaction
• Key files: skills/mach6-implement/SKILL.md, skills/mach6-push/SKILL.md, skills/mach6-review/SKILL.md