ci: add AI PR review workflow

[margaretjgu]

Summary

Adds an AI PR review workflow using the existing LITELLM_API_KEY secret (already provisioned for auto-pr.yml).

What it does:

• Runs on every non-draft PR open/update
• Sends the diff (capped at 32KB) to Claude via the LiteLLM gateway
• Posts a review comment focused on real bugs, missing error handling, logic errors, security issues
• Deletes stale bot comments on re-runs so the thread stays clean

What it skips: style nits, summaries of what the code does, anything not actionable.

No new secrets or provisioning needed - reuses LITELLM_API_KEY already in the repo.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	1		0	0	0.3s
✅ COPYPASTE	jscpd	yes		no	no	0.94s
✅ REPOSITORY	gitleaks	yes		no	no	57.51s
✅ REPOSITORY	git_diff	yes		no	no	0.43s
✅ REPOSITORY	secretlint	yes		no	no	27.32s
✅ REPOSITORY	trivy	yes		no	no	18.36s
✅ YAML	yamllint	1		0	0	0.46s

Notices

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