[FEATURE]: /verify — run local test and lint pipeline to validate repo state

Requested feature

Add a /verify slash command (inspired by Claude Code's /verify) that runs the local test/lint pipeline to validate the current state of the codebase.

Why

There is no single in-app action to validate repo health. Verification currently requires shelling out to bun test/lint manually, and there is ongoing work to wire TUI unit tests into CI (#41876).

Proposed behavior

• /verify discovers and runs the project's test + lint commands (package.json scripts, e.g. bun typecheck, bun test, lint).
• Streams results inline and reports pass/fail per step; exit code reflects overall result.
• Optional scope (files changed in the current session).

Benefits

• One-shot repo validation before committing or opening a PR.
• Rounds out the workflow: /verify → /review → /diff.