ci: add pull request quality gate

[andrei-hasna]

Summary

• add a single GitHub Actions job for pull requests and pushes to main
• run frozen root and dashboard installs before typecheck, build, and the full Bun test suite
• correct the stale CLI help assertion from webhooks to the current channels command

The dashboard install is separate because the root lockfile does not install dashboard dependencies, while the required build invokes Vite from that directory. No test suites are excluded.

Verification

• bun install --frozen-lockfile
• cd dashboard && bun install --frozen-lockfile
• bun run typecheck
• bun run build
• bun test (237 passed across 22 files)

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