[jefft72]

Summary

Gives AgentSessionRuntime the ownership model plan continuations need: exclusive runtime operations, a serialized prompt preparation queue, and a plan continuation entry point that cannot interleave with session replacement.

Replaces #65, which imported five modules that exist only on the task profile stack.

Reasoning

An accepted plan is submitted as a trusted prompt, and it must not race a /clear, /resume, or /fork that is already underway.
Serializing prompt preparation behind a single exclusive owner makes that race impossible rather than unlikely, and lets disposal wait for the in-flight operation to settle instead of tearing down under it.

Profile routing is deliberately absent.
The original PR interleaved harness resolution with plan activation inside the same methods, which is what coupled the two features.
Keeping the runtime plan-only here is what lets plan mode merge independently of the task profile stack.

Changes

• Adds exclusive runtime operation ownership with an idle assertion and settlement tracking
• Adds a prompt preparation queue so prompts and plan continuations admit in order
• Adds promptPlanContinuation, which takes exclusive ownership and validates the continuation is still pending
• Carries replacement diagnostics across session swaps
• Adds SessionImportFileNotFoundError and target validation for import and resume

Testing

• npx vitest --run test/suite/agent-session-runtime.test.ts test/suite/agent-session-plan-mode.test.ts
• tsgo --noEmit reports zero errors
• biome check . clean

[Issacboy123]

LGTM! Plan continuations now run through the runtime's normal ownership and replacement path, including forks and routed sessions. That gives them the same admission guarantees as other prompts. No issues discovered during review.