refactor(builder)!: reduce Builder to private Agent revisions

[Yevanchen]

Parent

• feat(platform): pivot Mosoo to an API-first managed Agent runtime #546

What to build

Shrink Agent Builder into an optional Workspace configuration surface for the durable Session API.

The remaining Builder may only:

• Create a Workspace-private Agent configuration.
• Edit fields actually consumed by Session execution.
• Create and inspect immutable revisions.
• Copy or select an exact revision reference for API use.
• Show the managed Harness, instructions, Skills, tools/MCP declarations, and permission configuration contained by a revision to the extent those fields exist in the final execution contract.

Remove:

• Publish and Unpublish.
• Draft versus Live state.
• Public/private visibility controls.
• Deployment or activation state and deployment history.
• Marketplace/gallery distribution behavior.
• Import, Export, Package, Manifest, and Fork affordances.
• Pet/Cattle Type selection and comparison.
• Any requirement to visit Builder before making a useful API request.

Builder is a thin console client of the same Workspace Agent-revision API used by SDK callers. It must not own a second lifecycle or duplicate execution semantics.

Managed Codex and Claude Code slugs remain directly callable without Builder. A private revision becomes callable by its owning Workspace immediately after creation. Preview or test actions, if retained, create an ordinary durable Session through the same Session API and appear in the same operational Session view.

Billing, balances, subscriptions, invoices, and BYOK settlement are non-goals.

Acceptance criteria

• A new Workspace completes a managed Codex or Claude Code Session without opening Builder.
• Builder creates a private Agent configuration and immediately exposes its immutable revision reference for API use.
• Editing a configuration creates a new immutable revision and does not mutate Sessions pinned to an earlier revision.
• A private revision is executable immediately without Publish, Unpublish, Draft, Live, deployment, or visibility transitions.
• Publish, Unpublish, Draft/Live, visibility, deployment, marketplace, Type, Package, Import/Export, and Fork UI and supporting reachable server contracts are removed rather than hidden.
• Builder displays only configuration fields consumed by the final Session execution snapshot.
• Any retained Preview/Test action executes through the ordinary Session API and appears in the same operational Session view.
• Generated clients, documentation, analytics events, tests, and empty states no longer describe the removed lifecycle.
• An end-to-end test creates revision 1 in Builder, runs it through the Session API, creates revision 2, and verifies both Sessions remain pinned to their selected revisions.
• No billing or BYOK settlement work is introduced.

Blocked by

• refactor(agent)!: remove packages, manifests, and fork #583 — Builder must target the final thin configuration/revision model after the package lifecycle is removed.