Add XCUITest target with accessibility identifiers

[hewigovens]

Summary

• Adds JayJayUITests target (new scheme JayJayUITests — won't slow down just test-app) with five scene tests: Command Palette, Interdiff, File Annotate, Review & Split, Conflict Resolution.
• Sprinkles .accessibilityIdentifier() on the views the tests need to target: palette text field, DAG rows (keyed by change-id prefix), file list rows (keyed by path), conflict-bar "Use Ours" / "Use Theirs" buttons. Strings centralized in Sources/JayJay/Shared/AccessibilityIdentifiers.swift.
• SceneBase reads the fixture repo path from /tmp/jayjay-demo-fixture (the XCUITest runner is sandboxed away from $HOME, so we can't use ~/). JAYJAY_DEMO_REPO env is honored as fallback.
• Adds just test-ui top-level recipe and shell::ui-test [test_id] — no test id runs the whole bundle.
• CI: new steps in the swift job clone a colocated jayjay fixture into /tmp and run just test-ui.

Test plan

• just build passes
• just test-app still runs only unit tests (UI tests isolated in their own scheme)
• xcodebuild -scheme JayJayUITests build-for-testing succeeds
• testOpenAndSearch passes locally (verified 16s runtime against a fixture clone)
• CI green on macos-26 runner — this PR is primarily to see what CI does with the full bundle

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive UI testing suite for the macOS application. It includes the addition of accessibility identifiers across various views (DAG, File List, Conflict Resolution, and Command Palette) to facilitate UI automation, new just commands for running UI tests, and several XCUITest scenes covering key features like annotation, command palette search, and conflict resolution. Feedback focuses on improving test reliability and performance by replacing hardcoded pauses with expectation-based waiting and optimizing element queries to avoid expensive tree traversals.