Do not link removed pull request files

[ymichael]

What was wrong

FileDiffCard rendered every pull-request file path as a live workspace link whenever the thread had an environment, without considering file.status. A removed file normally no longer exists in that workspace, so its filename advertised a preview target guaranteed to fail even though the separate diff control already exposed the deletion. This confirms the post-merge review finding.

What changed

• Reused the existing non-interactive filename rendering when file.status is removed, while retaining experimental_FileLink for statuses whose path can exist.
• Added one focused thread-panel regression covering the complete behavior: removed paths are not links, modified paths remain links, and the removed file's accessible expand/collapse control still reveals its diff.
• Kept the fix to the existing render condition; no abstraction, wire change, protocol bump, CLI, guide, or documentation change was needed.

How you verified

• Before the implementation change, pnpm exec turbo run test --filter=bb-plugin-github --force -- app.test.tsx failed the new regression with removed.ts rendered as <a href="removed.ts"> (1 failed, 2 passed).
• After the change, that focused command passed all 3 tests.
• After rebasing onto current origin/main, pnpm exec turbo run test --filter=bb-plugin-github --force passed 6 files / 25 tests.
• pnpm exec turbo run typecheck --filter=bb-plugin-github passed.
• pnpm exec turbo run build --filter=bb-plugin-github passed and built the GitHub plugin's server and app artifacts.
• pnpm exec prettier --check plugins/github/app.test.tsx and git diff --check origin/main...HEAD passed.

Fixes: no matching issue.

AGENT GENERATED: by GPT-5.6-Sol