[Feature]: Prevent pinned threads from auto-settling

[ImBIOS]

Problem

A sidebar pin (pinnedAt) is an explicit keep-active signal, but the server currently auto-settles pinned threads anyway:

• isAutoSettlementCandidate in apps/server/src/orchestration/ThreadSettlementPolicy.ts never checks pinnedAt, so pinned threads are swept for both inactivity and merged/closed-PR settlement.
• decider.ts thread.auto-settle has no pinned guard — it even emits a companion thread.unpinned event, silently dropping the user's pin.
• Docs (docs/user/thread-sidebar.md) state: "Pinning does not prevent automatic settlement."

The existing ThreadSettlementPolicy.test.ts case is even named "blocks pins, ..." but only covers settledOverride: 'active' (the transient keep-active override), not the sidebar pin.

Related:

• [Feature]: Durable per-thread "never settle" pin — the existing "active" override is cleared by activity, so no thread can be kept permanently active #5575 (durable never-settle override — closed, different primitive: settledOverride vs sidebar pinnedAt)
• fix(clients): move settled pinned threads into the settled section #7969 (moved auto-settled pinned threads into Settled — the behavior this reverses for auto-settle; manual settle still unpins and lands in Settled)

Proposal

• isAutoSettlementCandidate returns false when pinnedAt != null (covers inactivity + PR paths, skips PR lookups).
• thread.auto-settle decider rejects when pinnedAt != null (stale-sweep race: pin landed after the snapshot), while thread.settle (manual) still succeeds and clears the pin.
• Docs updated: pinning prevents automatic settlement.

Acceptance

• Inactive pinned thread never auto-settles
• Merged/closed-PR pinned thread never auto-settles
• Unpinning re-arms normal auto-settle rules
• Manual settle of a pinned thread still works and unpins

[juliusmarminge]

Triage

Sidebar pin (pinnedAt) is an explicit keep-active signal, but the server still auto-settles pinned threads (inactivity and merged/closed PR) and thread.auto-settle emits a companion thread.unpinned, dropping the pin. Current docs say pinning does not prevent automatic settlement.

This is real and matches main. It is an enhancement (documented, intentional today), not a defect.

Code

• isAutoSettlementCandidate in apps/server/src/orchestration/ThreadSettlementPolicy.ts never checks pinnedAt, so pinned threads are swept for both inactivity and PR settlement. The reactor filters on that helper before PR lookups.
• decider.ts thread.auto-settle guards settledOverride only. The shared settle path always unpins.
• The policy test named “blocks pins, …” only covers settledOverride: 'active' (transient keep-active), not sidebar pinnedAt.
• Web/mobile already bucket settledOverride === "settled" before pin. If the server never auto-settles a pinned thread, it stays in Pinned with no client change. Manual settle still unpins and lands in Settled.

settledOverride: 'active' and sidebar pinnedAt are different primitives. Activity clears the former; the latter is durable until unpin or settle.

Related

• Companion PR (same author, Fixes #11711): feat(settings): pinned threads stay active unless auto-settle pinned threads is on #11712 — do not open another.
• [Feature]: Durable per-thread "never settle" pin — the existing "active" override is cleared by activity, so no thread can be kept permanently active #5575 (converted to [Feature]: Durable per-thread "never settle" pin — the existing "active" override is cleared by activity, so no thread can be kept permanently active #6898) asked for a dedicated durable never-settle. Not a duplicate; this proposal reuses the existing sidebar pin.
• fix(clients): move settled pinned threads into the settled section #7969 moved auto-settled pinned threads into Settled on clients. This reverses auto-settle for pinned threads only; manual settle still unpins.

Next step

Review/merge #11712. Policy + decider + reactor tests and docs match the acceptance list. Unpinning re-arms auto-settle by clearing pinnedAt (no extra work). Pinning a settled thread already un-settles it, so treating pin as keep-active is consistent with pin-as-promotion.

Not a duplicate of #5575. No second implementation PR needed.