chore: add CLAUDE.md with contribution guidelines

[rubambiza]

Summary

• Adds CLAUDE.md documenting DCO sign-off, commit attribution policy, PR attribution, and scope discipline
• Documents how to install the commit-msg hook added in feat: add optional version field to bug report template #75

Closes #78

Test plan

• Verify git config core.hooksPath scripts/hooks activates the hook
• Verify Claude Code picks up the CLAUDE.md instructions in this repo

Assisted-By: Claude Code

[esnible]

Clean, docs-only PR adding CLAUDE.md to this repo. Verified all factual claims against upstream main:

• commit-msg hook exists at scripts/hooks/commit-msg ✓
• Referenced PR #75 is merged ✓
• The git config core.hooksPath scripts/hooks install instruction is correct for this repo (no .pre-commit-config.yaml here, unlike the main kagenti repo which uses pre-commit install) ✓
• The hook's emitted Assisted-By trailer matches the Commit Attribution section ✓

Areas reviewed: Docs / Markdown, accuracy of cross-references.
Commits: 1, signed-off: yes. CI: passing (DCO, add-to-project).

One nit inline; not blocking.

Assisted-By: Claude Code