feat: add webhook, k8s, and attachments gc CLI subcommands
Add admin CLI subcommands for managing webhooks, K8s job handlers,
and running attachment garbage collection via the Unix admin socket.
Server-side:
- Add WebhookServiceProvider and K8sServiceProvider interfaces to admin pkg
- Add 7 new command handlers: webhook.{register,list,delete},
k8s.{register,list,delete}, attachments.gc
- Wire webhook and k8s services into admin.Services struct in main.go
CLI-side:
- Add `synapbus webhook {register,list,delete}` commands
- Add `synapbus k8s {register,list,delete}` commands
- Add `synapbus attachments gc` command
- All commands follow existing patterns (adminRequest, printTable, printJSON)
Tests:
- Add cmd/synapbus/admin_test.go with 9 tests covering command
registration, required flag validation, and existing command preservation
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
