feat(desktop): add PR workspace and UI Lab

[IchenDEV]

Canonical change Artifact

• Change: docs/sdlc/changes/2026-09-01-pr-workspace-and-dock/change.md
• Related: docs/sdlc/changes/2026-09-01-ui-lab/change.md, docs/sdlc/changes/2026-09-01-floating-pr-inspector-prototype/change.md, docs/sdlc/changes/2026-09-01-floating-pr-inspector/change.md
• Schema: 2
• Lifecycle status: verified
• Intent/Spec approval: user requests and visual direction approved on 2026-09-01
• Risk and scope: medium; desktop PR workspace, PR Dock surface, development-only UI Lab, layout contracts, translations, tests, and the four canonical change bundles

Outcome

• Recompose Pull requests into a list, primary detail workspace, and contextual floating Inspector.
• Add a dedicated current-branch PR surface beside conversations while leaving Git focused on working-tree state.
• Add a permanent development-only UI Lab that renders production PR components with deterministic fixtures.
• Preserve review, merge confirmation, task linking, provider boundaries, and the 960px/704px responsive behavior.

Verification

• Every AC-N acceptance criterion is mapped to one actual command or linked evidence item.
• The Artifacts record Verdict: and Residual risk: consistently with their status.
• Verification mode, verifier, and date match the risk lane.
• cd apps/desktop && bun test — 815 passing tests, 0 failures, 3,909 expectations after merging current origin/main.
• cd apps/desktop && bun run build:renderer — ESLint, Stylelint, TypeScript, and Vite build passed.
• bun test script/verify/checks.test.ts, docs, SDLC, worktree, and whitespace Gates passed.
• Dark, light, 920px, and compact rendered checks passed without relevant errors or horizontal overflow.
• The packaged C2 Dev host launched against isolated data; same-host native WKWebView snapshots verified dark/light/narrow rendering and Review changes interaction while the Mac was locked.

Residual risk: macOS blocks pixel capture of the actual Electrobun window while locked, so native acceptance separates packaged-host startup from native-WKWebView visual rendering. A live authenticated GitHub mutation was not performed; existing mocked regression coverage preserves that boundary.

Review and release

• Risk level: medium
• Release target: none
• Rollback: revert this PR; there is no data migration or remote cleanup.
• Merge approval: pending
• Release approval: not applicable
• This PR does not infer merge, deployment, or release authority from implementation work.