Settle stranded runs interrupted during the admission window

[TheGreatAxios]

Summary

• The pending_init interrupt branch drops the run-in-flight marker alongside the lifecycle flip, matching every other terminal transition
• The spawn_agent not-admissible early returns settle through the normal terminal path (settleRun plus end telemetry) instead of only releasing the admission slot
• A store-level invariant in mutate drops a stranded run-in-flight marker whenever a terminal lifecycle has no settle-capable handle left

Verification

• bun test src/subagent/ passes (556 pass, 0 fail)
• bun run check passes (6980 pass, 0 fail)
• New regression covers the exact CL-7787 ordering: markRunInFlight, interrupt in the admission window, worktree resolves, wait status reaches terminal and the fleet goes dry

Fixes CL-7787

[linear-code]

CL-7787