Document edge cases for /run-skipped-ci workflow (#2129)

## Summary

- Documents edge cases for the `/run-skipped-ci` workflow in
CONTRIBUTING.md
- Adds important notes about force-push and branch deletion behavior

Closes #2002

## Test plan

- [x] Documentation only change - no functional changes
- [ ] Verify markdown renders correctly in GitHub

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Added "Important Notes" to contribution guidelines covering CI
behavior: how force-pushes can affect runs and a warning against
deleting or force-pushing branches while workflows run.
* Expanded developer install guidance for testing the generator:
detailed steps to link the local library, run the installer in the test
app, and publish/add the npm package locally to ensure proper
installation and linking for tests.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude <noreply@anthropic.com>

Author: justin808

CONTRIBUTING.md

@@ -536,6 +536,11 @@ Removes the `full-ci` label and returns to standard CI behavior:
 
 **Note:** The `full-ci` label is preserved on merged PRs as a historical record of which PRs ran with comprehensive testing.
 
+#### Important Notes
+
+- **Force-pushes:** The `/run-skipped-ci` command adds the `full-ci` label to your PR. If you force-push after commenting, the initial workflow run will test the old commit, but subsequent pushes will automatically run full CI because the label persists.
+- **Branch operations:** Avoid deleting or force-pushing branches while workflows are running, as this may cause failures.
+
 ### Install Generator
 
 In your Rails app add this gem with a path to your fork.