refactor: rename SowerAgent to Garden
Had Claude use a script, roughly, and then do some manual cleanup.
refactor: rename Agent → Field in database & internal domain model
refactor: rename Agent → Field in channel contract with dual-accept
refactor: rename Agent → Field in web UI and routes
refactor: rename Agent → Field: internal cleanup (non-compat)
refactor: rename Field → Garden across codebase
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>