feat: compact pr-comment format for GitHub Action PR comments

[KryptosAI]

Summary

• Adds --format pr-comment — a compact, severity-tiered output designed for GitHub PR comments
• 10 lines vs 146 for the current --format markdown (93% reduction)
• Updates the GitHub Action to use pr-comment format by default
• Every CI run now produces a scannable, eye-catching comment that PR reviewers actually read

What it looks like

When issues found:

## 🔭 MCP Observatory — 1 issue found

### ⚠️ QUALITY (1 warning)
> prompt "daily-brief": Argument 'topic' missing description

### 📊 Summary
> Health: **B** (82) · 1 tools · 1 prompts · 2 resources

---
<sub>🔭 MCP Observatory — test your MCP servers for breaking changes · ⭐ Star</sub>

When all clear:

## 🔭 MCP Observatory — All clear ✅

All checks passed. No security issues, no schema quality warnings.

### 📊 Summary
> Health: **A** (95) · 24 tools · 3 prompts · 5 resources

Design

• Tiered output: Security (🔴) → Failing (🔴) → Quality (⚠️) → Summary (📊)
• Max 5 items per section with "...and N more" truncation
• Handles both RunArtifact (single scan) and DiffArtifact (regression comparison)
• Diff mode shows: Regressions → Schema Drift → Recoveries → Summary
• Branded footer with repo link and star CTA

Files changed

File	Change
src/reporters/pr-comment.ts	NEW — 195-line renderer
src/index.ts	Added export
src/commands/helpers.ts	Added format dispatch
src/commands/legacy.ts	Added format option
src/commands/diff.ts	Added format option
action/action.yml	Switched to pr-comment, removed double-wrapping
tests/pr-comment.test.ts	NEW — 14 tests

Test plan

• npm run build — clean compile
• npx vitest run — 236/236 tests pass (14 new)
• Manual test: mcp-observatory report <artifact> --format pr-comment against fixture server
• Verified output is <30 lines for typical scans
• CI pipeline validates on push

🤖 Generated with Claude Code

[github-actions]

🎉 This PR is included in version 0.19.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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