ci: cache and cancel stale GitHub Actions runs (#105)

## Summary

Keep CI on pull requests and `main` while caching Bun packages and
cancelling superseded validation, release-preview, and comment runs.

## Workflow behavior

| Workflow | Execution policy |
| --- | --- |
| CI | Runs on pull requests, `main`, and merge-queue groups; cancels
superseded runs in the same group |
| Publish | Remains serialized on `main`; preserves release and npm OIDC
permissions |
| Tegami PR | Cancels obsolete PR previews and retains the handoff
artifact for seven days |
| Tegami PR Comment | Keeps the trusted `workflow_run` comment boundary
and cancels obsolete comments |

## Validation

- Workflow YAML parsed successfully.
- `bun run typecheck:examples`
- Docs reference, typecheck, snippets, build, registry, link, and audit
checks passed.
- `bun run release:check` reached the React browser suite. Seven browser
tests passed before its local fixture server returned
`ERR_CONNECTION_REFUSED` for the final four tests. Recent GitHub CI runs
passed this suite.