Keep plugin host calls responsive across environment changes

[ymichael]

Human comments

What was wrong

Plugin host calls share a process and its environment. After #3274, a call needing changed machine variables waited for all active calls to finish. That could block a stop/close call behind the long-running work it was meant to stop. Cancelling a waiting call could also trigger the manager's five-second force-kill timer and interrupt unrelated calls in the same worker.

What changed

Remove the environment wait queue. While any calls are active, incoming calls run immediately using the worker's current environment, even if they supply different values. Once all calls finish, the next incoming call supplies fresh values. Values from overlapping calls are not saved as pending updates. Continuous overlapping work can intentionally retain old settings until the worker becomes idle.

The environment is restored when the last active call finishes. Existing cancellation and force-kill protection for unresponsive handlers remain intact. Acquisition stays inside the call's error cleanup.

Bump HOST_DAEMON_PROTOCOL_VERSION from 205 to 206 so enrolled machines update for the changed environment semantics. Document when plugin calls adopt settings in the machine CLI guide, configuration docs, and bb-cli skill reference. No new CLI flags or public SDK members. This PR does not touch UI code.

How you verified

• Updated real-worker regression coverage for immediate admission with different values, retaining the first values until every overlapping call finishes, adopting the next idle call's values, variable removal, cancellation/deadlines, disposal, and hung-handler termination. Four tests failed with the previous waiting implementation; all 32 plugin host manager tests pass after the change.
• Post-rebase pnpm exec turbo run test build typecheck --filter=@bb/host-daemon --filter=@bb/host-daemon-contract --force — 617 daemon tests and 56 contract tests passed; all 11 Turbo tasks passed.
• Actual browser-plugin host handlers running through the real manager, worker process, and IPC, with an injected cooperative runtime: previously close waited behind a two-second run after a variable change; now it interrupts the run and returns in 1 ms with either unchanged or changed values.
• Verified through the actual built bb CLI, isolated source server, enrolled built daemon, and a diagnostic plugin installed with bb plugin install. Two 20-second calls overlapped across a machine-variable update and retained the old values in one worker PID; stop commands returned in 268 ms and 259 ms; the first idle call adopted the new values and later reflected removal.
• Targeted formatting and git diff --check origin/main...HEAD passed.

AGENT GENERATED