Migrate certain backend features into berd

The goose ACP+ server implements several methods that are unused by the GRC. Most are used by [Berd](https://github.com/block/berd); the table below records current production usage and the recommended ownership boundary.

Assessment criteria:

- If neither GRC nor Berd uses a method, delete it unless there is another confirmed client.
- If Berd uses a method that manipulates Goose-owned runtime or persisted state, keep it in Goose ACP.
- If Berd uses a method for Berd-owned application content or lifecycle, move it into Berd's Tauri backend.

Method | Findings | Decision
-- | -- | --
`_goose/unstable/dictation/models/select` | No production usage found in GRC or Berd. | **delete (done)**
`_goose/unstable/dictation/secret/delete` | No production usage found in GRC or Berd. | **delete (done)**
`_goose/unstable/dictation/secret/save` | No production usage found in GRC or Berd. | **delete (done)**
`_goose/unstable/extensions/available` | No production usage found in GRC or Berd. | **delete(done)**
`_goose/unstable/onboarding/import/apply` | Used by Berd for first-boot migration; this is Berd application lifecycle behavior. | **migrate to berd**
`_goose/unstable/onboarding/import/scan` | Used by Berd for first-boot migration; this is Berd application lifecycle behavior. | **migrate to berd**
`_goose/unstable/preferences/remove` | No production usage found in GRC or Berd. | **delete(done)**
`_goose/unstable/providers/config/status` | Used by Berd to read Goose provider configuration state. | **keep**
`_goose/unstable/providers/setup/catalog/list` | Used by Berd; Goose owns the provider setup catalog and behavior. | **keep**
`_goose/unstable/providers/supported-models/list` | Used by Berd for model caching and model-selection repair; Goose owns provider model discovery. | **keep**
`_goose/unstable/session/archive` | Used heavily by Berd for chat archival and cleanup. GRC adoption is tracked in https://github.com/aaif-goose/goose/issues/11057. It mutates Goose session persistence. | **keep**
`_goose/unstable/session/project/update` | Used by Berd to associate sessions with Berd projects; project association is Berd-owned metadata. | **migrate to berd**
`_goose/unstable/session/system-prompt/set` | Used by Berd for security/background sessions and persona handoff; it mutates live Goose agent state. | **keep**
`_goose/unstable/session/unarchive` | Used by Berd to restore archived chats. GRC adoption is tracked in https://github.com/aaif-goose/goose/issues/11057. It mutates Goose session persistence. | **keep**
`_goose/unstable/sources/create` | Used by Berd for skills, projects, and personas. These are Berd-owned application content and CRUD can live in its Tauri backend. | **migrate to berd**
`_goose/unstable/sources/delete` | Used by Berd for skills, projects, and personas. These are Berd-owned application content and CRUD can live in its Tauri backend. | **migrate to berd**
`_goose/unstable/sources/export` | Used by Berd for skill export. | **migrate to berd**
`_goose/unstable/sources/import` | Used by Berd for skill and persona import. | **migrate to berd**
`_goose/unstable/sources/update` | Used by Berd for skills, projects, and personas. These are Berd-owned application content and CRUD can live in its Tauri backend. | **migrate to berd**