chore(devkitd): drop legacy agents.db bridge after full migration #416

Part of the devkitd agent migration (#406). Endgame cleanup — lands LAST, once zero consumers read agents.db.

Scope

• Delete import_legacy_agents (boot delta-merge)
• Delete dual_write_legacy + legacy_db plumbing in store/registry.rs
• agents.db becomes purely legacy; TS AgentRegistry no longer consulted by any command

Acceptance

• All agent/channel/group paths are daemon-first with no agents.db reads
• store/registry.rs contains only the live registry