test: cover plan runtime routing races - #67

Summary

Expands runtime and profile-routing coverage for native plan mode, accepted implementation handoff, exclusive replacement ownership, and recovery from failures at every asynchronous switch boundary.
Adds characterization tests showing when implementation should select a harness, stay in an existing or pinned scope, retain its continuation for retry, or leave the outgoing session untouched.

Reasoning

An automatic handoff can cross routing, confirmation, switch hooks, candidate creation, shutdown, invalidation, activation, and host rebinding before a prompt reaches the model.
Tests need to prove that no queued or competing prompt crosses that boundary, that pre-shutdown failures remain reversible, and that post-shutdown failures still leave one adopted runtime.

Plan lifecycle state makes automatic resume and tool suppression part of the same contract.
A routed session must never consume another transcript's unresolved plan, and SDK tool restrictions must remain effective when collaboration mode changes.

Changes

• Verifies that plan-mode prompts bypass both initial and future automatic profile routing and remain in the current session and profile.
• Covers routing an accepted plan before its implementation turn, preserving the existing session manager and file, and recording exactly one trusted continuation message in the activated harness.
• Confirms that already task-scoped harnesses and explicitly pinned profiles implement accepted plans in place without rerouting or creating another harness.
• Exercises preflight and harness-activation failures so the correct original or rebound continuation remains retryable and a successful retry does not repeat completed routing.
• Proves that accepted-plan routing excludes competing prompts and replacements, while disposal waits for routing to settle and prevents late activation after shutdown begins.
• Covers races where plan mode starts during routing, confirmation, a switch hook, or candidate creation, ensuring the prompt stays in its original session, is delivered once, and routing ownership is released for a later retry.
• Verifies that queued pre-switch prompts never enter a rebound session and that stale outgoing-session mutations remain blocked throughout shutdown and invalidation.
• Exercises candidate-creation and related-session precommit failures before shutdown, plus invalidation and activation failures after shutdown, to distinguish reversible handoffs from one-way transitions and keep a usable runtime in both cases.
• Rejects automatic harness resume when the target session is in plan mode or contains an accepted continuation, preserving the current transcript and active profile.
• Extends runtime characterization for target validation, independent in-memory forks, failed fork creation, delayed rebinding, and noTools behavior so plan mode keeps only intrinsic submission when built-ins are disabled.

Testing

• Final rebased stack: npm run check passed.
• Final rebased stack: focused Vitest validation across 28 affected files passed all 451 tests.
• This intermediate branch was not checked independently after reconstruction.