feat: GitLab merge request review support

[backnotprop]

Summary

• Full GitLab MR review parity with existing GitHub PR review
• Auto-detects platform from URL — github.com routes to gh, any /-/merge_requests/ URL routes to glab
• Self-hosted GitLab instances supported via --hostname flag
• Platform-aware UI throughout: labels (PR/MR), icons (GitHub/GitLab), number prefixes (#/!), completion messages

Architecture

• Extracted GitHub logic into packages/shared/pr-github.ts
• New packages/shared/pr-gitlab.ts with full glab CLI implementation
• Widened PRRef/PRMetadata to discriminated unions ("github" | "gitlab")
• Dispatch functions in pr-provider.ts route by platform
• Label helpers (getPlatformLabel, getMRLabel, getMRNumberLabel, getDisplayRepo) accept both ref and metadata types

GitLab-specific handling

• glab mr diff outputs bare diffs — normalized to standard diff --git format
• glab api requires explicit Content-Type: application/json header for --input -
• glab has no --jq flag — parse full JSON responses instead
• Review submission uses 3 separate API calls (note + discussions + approve) vs GitHub's single atomic review endpoint
• Inline comments submitted in parallel with Promise.allSettled for partial failure resilience
• Defensive JSON parsing on all context API responses (notes, approvals, pipelines, issues)

Files changed

File	Change
packages/shared/pr-provider.ts	Widened types, dispatch functions, label helpers
packages/shared/pr-github.ts	New — extracted GitHub implementation
packages/shared/pr-gitlab.ts	New — full GitLab implementation
packages/server/pr.ts	Updated Bun wrapper for dispatching
packages/server/review.ts	Platform-agnostic endpoint handling
apps/hook/server/index.ts	Platform-aware error messages and status
apps/opencode-plugin/commands.ts	Same platform-aware updates
packages/review-editor/App.tsx	Platform-aware UI (labels, icons, destination toggle)
packages/review-editor/utils/exportFeedback.ts	Platform-aware export headers
packages/ui/components/GitLabIcon.tsx	New — GitLab tanuki SVG icon

Test plan

• All 267 existing tests pass
• GitHub PR review regression tested (feat: bidirectional AI conversation, context persistence, and approve-with-notes #317)
• GitLab MR review tested (backnotprop1-group/backnotprop1-project!1)
• Inline comments post to correct lines on GitLab
• Multi-line range comments work
• Platform detection from URL works for both
• Diff normalization renders multi-file GitLab diffs correctly
• Completion overlay shows correct platform name