Add validateRecipient() to check that mail recipients correspond to
existing agents before sending. This prevents mail from being stored
with invalid assignees that won't match inbox queries.
The validation queries agent beads and checks if any match the
recipient identity. The only special case is "overseer" which is the
human operator and doesn't have an agent bead.
Tests create a temporary isolated beads database with test agents
to validate both success and failure cases. Tests are skipped if
bd CLI is not available (e.g., in CI).
Fixes gt-0y8qa
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
4e4824a6c6