feat(subagents): add dismiss tool for stale running entries

[interpreteragent]

Summary

Adds a subagent_dismiss tool for cleaning up stale subagent status entries from the parent session.

This covers cases where the child pane/session is killed outside the normal lifecycle, or a child gets stuck in a non-standard waiting/stalled state and never produces the normal sentinel/result. Today the widget can keep showing that child forever, with no parent-side way to forget it.

The new tool:

• removes the selected running subagent from runningSubagents
• aborts its watcher so no later subagent_result is emitted for the dismissed entry
• updates the widget immediately
• optionally closes the pane via closePane: true

I also added subagent_dismiss to the spawning: false denied tool set so child agents do not get parent-side subagent management tools.

Example

subagent_dismiss({ name: "Worker" })
subagent_dismiss({ id: "abcd1234", closePane: true })

Test plan

• Added unit coverage for tool registration.
• Added unit coverage for dismissing with and without closing the pane.
• Ran bun build for pi-extension/subagents/index.ts and pi-extension/subagents/subagent-done.ts.
• Ran bun build for test/test.ts.

No package-lock changes included.