fix(session): answer prompts that land while the previous turn is finishing

[filipeforattini]

Problem

Runner.ensureRunning in the Running state discarded the incoming work and only awaited the current run. SessionPrompt.prompt writes the user message straight into history and then calls loop(), so a prompt that landed after the loop's final history read but before finishRun flipped the state to Idle (the window includes lastAssistant(), compaction.discard, and the Turn.Ended hooks/events) was persisted but never answered. The session went idle with an unanswered trailing user message until the next prompt.

Fix

Runner-level, per CONTEXT.md ("the runner promotes one queued prompt when the Session would otherwise become idle, then reevaluates continuation before promoting another"):

• Running gains an optional pending handle. ensureRunning while running records the arrival (latest work thunk, one shared done deferred) and returns an await on that deferred.
• finishRun starts the pending run instead of going idle when the finishing run was not interrupted. onIdle is not fired between the two runs, so status does not flicker.
• A cancel already leaves Running before the interrupted fiber exits, so it never restarts; it now also fails the pending deferred so callers that arrived during the run resolve through onInterrupt. A run interrupted any other way (scope teardown) drops the pending work rather than starting work in a dying scope.

Test

• test/session/prompt.test.ts: "answers a prompt that lands while the previous turn is finishing" holds the first turn open in a Turn.Ended hook, sends a second prompt in that window, and asserts it gets its own reply, the history ends with an assistant message answering the last user message, the session is idle, and the fake provider has no unconsumed reply. Fails before the fix (second prompt resolved with the first turn's reply), passes after.
• test/session/prompt.test.ts: "cancel while a turn is finishing does not start another run" cancels inside the same window and asserts exactly one Turn.Ended, the second reply is never requested, and the session is idle.
• test/effect/runner.test.ts: three new cases (follow-up run before idle, callers sharing one follow-up run, cancel dropping pending work) fail before and pass after; two existing cases that encoded the old "silently join" semantics are updated to the new contract.
• test/session directory: 514 pass, 0 fail. bun run typecheck clean.

https://claude.ai/code/session_01KAcoL6wpgVEs2ebUrUGpHS

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