feat: PR review commands and display (NEX-002)

[flyingrobots]

Summary

Add markdown formatting for PR suggestion display and /gitmind slash command handling in GitHub Actions.

Scope

New files

• src/format-pr.js — Markdown table formatter (~80 LoC)
• test/format-pr.test.js — Tests (~80 LoC, ~8 tests)
• .github/workflows/gitmind-review.yml — Slash command handler workflow

Modified files

• action/post-comment.js — Use formatSuggestionsAsMarkdown from format-pr.js
• src/index.js — Export new functions
• CHANGELOG.md

Key functions

formatSuggestionsAsMarkdown(suggestions)  // → markdown table string
parseReviewCommand(body)                  // → { command, index } | null

PR comment format

Markdown table: #, Source, Target, Type, Confidence, Rationale
Footer: /gitmind accept 1 | /gitmind reject 2 | /gitmind accept-all

Part of NEXUS milestone.