feat(web): stop active subagents from Lineage

[Bil0000]

Lineage shows active subagents and their elapsed time, but gives no way to stop one there.

Add a stop button over the timer on hover or keyboard focus. It interrupts T3-owned child runs and native Codex child turns without opening the child thread. The button is hidden for settled agents and native providers without a targeted stop path.

Verified with 77 focused tests, type checks for the server, web, client runtime, and contracts, plus targeted lint and formatting.

Limit: Native per-child stop is available for Codex only. The after image uses fake local subagent data.

Before and after

Before (reported Lineage timer):

After (stop button on hover):

Model: GPT-6 in the native Codex harness.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds a new Lineage stop workflow that can interrupt active app-owned runs or native Codex provider turns through new cross-layer command, orchestration, and effect plumbing. The asynchronous provider-control side effects and shared infrastructure changes warrant human review despite additive contracts and focused tests.

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