feat: Improve PR comment UX - show resolved conflicts and new-conflict badges

[zkoppert]

Closes #49

What it does

Improves the PR comment experience by making the bot's comments more informative and transparent about how conflicts evolve over time.

Changes

1. Auto-update banner - Adds a note at the top of every comment: "🔄 This comment updates automatically as conflicts are detected and resolved." This sets expectations that the comment is live.

2. 🆕 badge on new conflicts - Newly detected conflicts get a 🆕 indicator in the table. On the next run (if still present), the badge is removed. Gives authors a visual cue for what changed.

3. Collapsed resolved conflicts section - Instead of silently removing resolved conflicts, they're moved to a collapsed <details> section with strikethrough formatting and the date they resolved. Preserves history without cluttering the main view.

4. All-resolved message - When all conflicts for a PR are resolved, the comment updates to show "✅ All Merge Conflicts Resolved" with the resolved history.

5. Bug fix: unchanged conflicts no longer dropped - Previously, post_pr_comments() only received new/changed conflicts. If a PR had 3 conflicts and only 1 was new, the comment update would show only that 1 conflict, dropping the other 2. Now all active conflicts are passed so every PR's comment shows its full picture.

Known limitation

The 🆕 badge only persists until the next action run. If the action runs multiple times before the author checks the comment (e.g., hourly on weekdays), the badge will already be gone by the time they look. This is acceptable for now - the resolved section and banner still provide meaningful context about how conflicts evolved. A future improvement could address this with a notification bump reply (see #49 for discussion). Open to ideas on how we might solve this. 👂🏻

State file changes

• ConflictFingerprint now stores PR display info (title, URL, author) so resolved conflicts can render meaningful details
• State file gains a resolved_conflicts list with resolved_at timestamps
• Resolved entries are pruned after 7 days
• Backward compatible: old state files without the new fields still load correctly

Example comment format

Active conflicts + resolved section

⚠️ Potential Merge Conflicts Detected

🔄 This comment updates automatically as conflicts are detected and resolved.

This PR may conflict with 2 other PR(s):

	Conflicting PR	Conflicting Files (Lines)
🆕	#456 (Refactor auth)	src/auth.py (L15-L20)
	#789 (Add logging)	src/app.rb (L10-L30)

What to do: Review the overlapping changes and coordinate with @bob, @charlie to resolve conflicts.

✅ 1 previously resolved conflict

Conflicting PR	Resolved
#100 (Fix typo)	Mar 19, 2026

This is an automated notification from pr-conflict-detector.

Testing

• 269 unit tests pass (99.2% coverage)
• All 5 linters pass clean (pylint 10/10)
• New tests cover: enriched fingerprints, resolved state tracking/pruning, new comment format with banner/badges/resolved section, all-resolved comment, flip-flop dedup, timezone-naive safety, backward compatibility with old state files

[zkoppert]

Files are getting long. I've opened an issue to do a refactor after this merges (don't want this pr review to be any more painful than it has to be).

[jmeridth]

Very nice. Easy to read code, good tests.