feat(server): safely evict idle directory instances - #42631

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found one potentially related PR:

Related PR:

• fix(opencode): evict idle per-directory instances to bound serve memory #33713 - fix(opencode): evict idle per-directory instances to bound serve memory
  • This PR also addresses idle eviction of per-directory instances for memory management. It may be an earlier or alternative approach to the same problem that PR feat(server): safely evict idle directory instances #42631 is solving.

The current PR (#42631) appears to be a more refined, safety-focused implementation with explicit opt-in through an experimental flag and comprehensive safety mechanisms, whereas #33713 seems to be a simpler fix addressing the same underlying issue.

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AI code review — automated review for reference; please use your judgment.

Scope: opt-in idle eviction of directory instances: a module-level activity tracker (per-directory generation counter + running-PTY counts), PTY start/stop hooks, a 1-minute sweep that disposes instances idle beyond a configurable timeout, and refactored instance-store leasing (provide() now issues explicit leases; markInstanceForDisposal/Reload become immediate disposeInstance/reloadInstance).

• The race discipline is the standout: eviction takes an activity snapshot, re-validates generation + zero-ref + cutoff after the async canDispose, and finally guards disposal behind isCurrentAndIdle(snapshot) in the same synchronous stack as the state transition. The comment documenting that sync-stack requirement is exactly right. PTY counting uses a counted flag so kill/exit paths can't double-decrement.
• Gated correctly: instanceIdleTimeoutMs undefined ⇒ no sweeper at all, plus a separate warning flag — safe rollout shape, and the flags parsing has its own tests.
• canDispose covers the right signals (running PTYs, active runners, non-idle session statuses, running background jobs); each check is individually testable since they're injected.
• Minor leak: InstanceActivity.states grows one entry per directory ever touched and forget() only runs on explicit paths — bounded by distinct directories per process, so harmless, but calling forget after successful eviction would keep it tidy.
• The global mutable generation counter means any instance's touch invalidates every other instance's snapshot — conservative (fails toward not evicting), fine at this scale, just noting it makes sweeps more no-op-ish under broad activity.
• Test coverage is exceptional (+552 lines of instance-store scenarios including mid-sweep races, disposing-state reacquisition, lease release on disposeAll) — this is what makes a concurrency feature like this mergeable.

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