feat: #13 - Exhaustive PR comment resolution with mandatory replies

[iamladi]

Summary

Rewrites /address-pr-comments to guarantee every PR comment receives a GitHub reply — no feedback silently skipped.

• Mandatory replies: Every non-author, non-bot comment gets a reply (fix with SHA, explanation, acknowledgment, clarification request, or deferral)
• Auto-commit per fix: Each code fix gets its own commit referencing reviewer and comment ID
• 3 comment types: Adds review summary fetching alongside review comments and issue comments, all with --paginate
• Safe reply posting: All reply bodies piped via jq --arg + --input - to prevent shell injection
• Idempotent re-runs: Detects existing replies before posting to prevent duplicates
• Line-drift safety: Uses diff_hunk anchor text instead of API line numbers
• Preflight checks: gh auth, clean working tree, git identity — with reasoning for each
• Constitution compliance: Scoring system uses judgment dimensions instead of point tables; reply templates replaced with quality criteria; fix workflow framed as principles

Plan

See plans/exhaustive-pr-comment-resolution.md — reviewed by GPT-5.2-Codex and Gemini 3 Pro (16 blindspot findings addressed).

Constitution compliance review: reviews/constitution-compliance-address-pr-comments.md (4.4/10 → transformed)

Closes #13

Test plan

• Run bun run validate — plugin validation passes
• Run /address-pr-comments on a PR with mixed comment types (code fix, question, praise, unclear suggestion)
• Verify all comments receive GitHub replies
• Verify each fix has its own commit with descriptive message
• Run command twice on same PR — verify no duplicate replies (idempotency)
• Test with dirty working tree — verify preflight abort
• Test autonomous mode (--auto) — verify below-threshold comments get replies