Allow machine removal while preserving read-only thread history

[ymichael]

Human comments

What was wrong

Explicit machine removal was blocked by unarchived threads, and persistent environment cleanup also prevented removing compute while retaining thread history. Threads whose machines had been removed could appear offline or errored and still expose controls that required the missing machine.

What changed

• Allow explicit removal for both disposable and persistent machines, stopping active work and preserving threads as read-only history. Retain the host row as a tombstone so historical references remain intact. Automatic cleanup still respects live threads.
• Propagate environment cleanup failures to machine removal and let Retry cleanup retry the failed environments before removing compute.
• Expose the retained machine name and removal state through thread runtime data, including existing SDK and CLI reads. Show consistent removing, cleanup failed, and removed states across the sidebar, workspace details, and read-only composer banners; hide unavailable execution controls.
• Record intentional interruption as host-removed, with “Stopped because the machine was removed” in the timeline. Bump HOST_DAEMON_PROTOCOL_VERSION to 217 for the shared wire schema change, following version 216 on main.
• Update machine-removal CLI guidance, skills, and verification documentation. The existing remove command uses the same behavior as the UI.

How you verified

• Passed pnpm exec turbo run typecheck --filter=@bb/server --filter=@bb/app, plus affected core UI, host daemon, thread view, and database typechecks.
• Passed focused server tests for public host management, runtime status, machine lifecycle recovery, and provider orchestration. Coverage includes active/stopping threads, retained history, persistent machines, cleanup failure, and retry.
• Passed focused app tests for removal dialogs/settings, sidebar states, read-only composer and banners, workspace display, and thread metadata.
• Passed affected database lifecycle, core UI display, thread timeline, and host daemon contract tests. All test runs used Turbo with package and file filters.
• Visually checked the three machine-removal banner states in the source Ladle story at desktop and narrow widths.
• git diff --check passed.

AGENT GENERATED