chore: add CLAUDE.md with contribution guidelines

CLAUDE.md

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+# CLAUDE.md - Kagenti .github Repository
+
+## DCO Sign-Off (Mandatory)
+
+All commits **must** include a `Signed-off-by` trailer (Developer Certificate of Origin).
+Always use the `-s` flag when committing:
+
+```sh
+git commit -s -m "feat: Add new feature"
+```
+
+This adds a line like `Signed-off-by: Your Name <your@email.com>` to the commit message.
+PRs without DCO sign-off will fail CI checks. To retroactively sign-off existing commits:
+
+```sh
+git rebase --signoff main
+```
+
+## Commit Attribution Policy
+
+When creating git commits, do NOT use `Co-Authored-By` trailers for AI attribution.
+Instead, use `Assisted-By` to acknowledge AI assistance without inflating contributor stats:
+
+    Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>
+
+Never add `Co-authored-by`, `Made-with`, or similar trailers that GitHub parses as co-authorship.
+
+A `commit-msg` hook in `scripts/hooks/commit-msg` enforces this automatically.
+Install it by configuring the hooks path:
+
+```sh
+git config core.hooksPath scripts/hooks
+```
+
+## PR Description Attribution
+
+When generating PR descriptions, summaries, or any PR metadata, use
+`Assisted-By: Claude Code` — never `Generated by Claude Code` or similar phrasing.
+The work is the developer's; Claude Code assists. This differs from the commit
+trailer format (`Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>`)
+because PR body text is human-readable and doesn't need the email for git tooling.
+This applies to:
+
+- PR body text (e.g., the footer line)
+- Commit message trailers referenced in PR descriptions
+- Any auto-generated changelogs or release notes
+
+## Scope Discipline
+
+Only commit files directly related to the PR's stated purpose. Do not bundle
+unrelated changes (tooling, configs, hooks, formatting) into a PR unless the
+PR is explicitly for that purpose. If a prerequisite change is needed, open a
+separate PR for it first.