Schema groundwork for v2 agentic-comms milestone 1.
mesh.message_queue gets three nullable columns (claimed_at, claim_id,
claim_expires_at) so drainForMember can move from "claim-and-deliver in
one UPDATE" to a two-phase claim/lease + recipient-ack model. This is
the at-least-once retry hook the broker has been missing.
mesh.presence gets a typed `role` column ('control-plane' | 'session'
| 'service') with default 'session' so legacy hellos keep working. The
CLI's hidden-daemon hack (peerType === 'claudemesh-daemon') will swap
to a role-based filter in a follow-up worktree.
Migration is hand-authored as 0029_*.sql to match the existing pattern
(drizzle-kit's _journal.json drifted long ago — the runtime migrator
in apps/broker/src/migrate.ts tracks files lexicographically via
mesh.__cmh_migrations, not the journal).
