Refactors the contacts system from being embedded in cal_tool.py into a
standalone contacts skill that serves as the single source of truth for
recipient validation across all outbound email paths.
- New skills/contacts/ skill: list, add, delete, resolve commands
- New skills/himalaya/scripts/himalaya.sh wrapper: validates To/Cc/Bcc
recipients against contacts for message send, template send, and
message write commands; passes everything else through unchanged
- cal_tool.py now delegates to contacts.py resolve instead of inline logic
- TOOLS.md updated: agent should use himalaya wrapper, not raw himalaya
