fix(buzz-acp): refresh idle clocks on turn completion and steer ack

[Stalker23b]

Fixes #7171

Problem

The idle-pool reaper clocks (idle-pool sleep, exit-after-inactivity) are refreshed only at dispatch time. Any turn that runs longer than the idle bound is guaranteed to look idle the moment it completes, so the first reaper tick after completion tears the pool down — killing the CLI process together with its live background tasks and pending wake-ups.

Observed in production: a 28-minute turn completed and the pool was reaped 13 seconds later, orphaning two background tasks whose completion the agent was waiting on (details in #7171).

Fix

Turn completion and steer acks now count as activity:

• The idle-clock refresh lives inside handle_prompt_result, so the existing unit tests around result handling also guard the wiring.
• The refresh decision is an exhaustive match on PromptSource: a new variant must explicitly decide whether its completion keeps the pool alive.
• Heartbeat results are excluded — heartbeat dispatch is gated on an idle slot with no flushable work, so counting them would keep a heartbeat-enabled pool awake forever.

Known edges (unchanged, noted for reviewers)

• The in-flight deadline is max_turn_duration + 100s; a turn exceeding it auto-expires and the pool can still be reaped before the turn ends (pre-existing).
• The panic path does not refresh the clocks (pre-existing).

Testing

• New unit tests: result_refreshes_idle_clock (mapping, exhaustive match) plus wiring tests that a channel result refreshes the idle clock through handle_prompt_result and a heartbeat result leaves it stale.
• cargo test -p buzz-acp: 28 failures on this (heavily loaded) machine, in modules this patch does not touch (acp::tests steer/session, pool::model_switch_tests, pool::startup_effort_tests) — the failure set is byte-identical on the clean base commit under the same conditions, i.e. pre-existing machine-local flakes, zero new failures from this patch. The four new idle-clock tests pass.
• Running in production across 18 agents since 2026-09-01: no premature teardowns after long turns.

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[github-actions]

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