Needs you is never buried in Settled: attention and tasks from other threads keep a thread active #137

Problem

A settled thread can hide a request for the user:

1. Another thread sends a settled thread a task (t3_thread_send). The message brings the thread back, as any message does, and the agent works on it.
2. When the run ends, automatic settlement settles it again at once. The thread has a merged pull request, and settle-on-merge (upstream #8600) only looks at messages the user wrote since the merge. A task from another thread doesn't count, so the merge still reads as "done".
3. If the agent asked for the user during that run (t3_request_attention, Experts: background turns stay quiet, and "Needs you" is an explicit call #101), its Needs you ends up in the Settled section, where nobody sees it. isAutoSettlementCandidate doesn't look at attention.

Fix

• Needs you stays visible. A thread with attention set is never auto-settled. t3_request_attention on a settled thread brings it back the way a message does: settledOverride and settledAt are cleared, it is not a user override.
• A task from another thread is new work. For settle-on-merge, a message another thread sent counts like the user's own (latest user or thread-sent message vs. mergedAt). Automatic deliveries stay excluded: notifications such as pull request watch wakes, delegated task completions, scheduled runs and provider wakes.
• Inactivity settlement (sidebarAutoSettleAfterDays) is unchanged.

Proof

Failing tests first, in ThreadSettlementService's existing test file:

• A thread with a merged pull request and a later message from another thread is not auto-settled.
• A thread with attention set is not auto-settled. Once attention is cleared, it settles as before.
• A merged pull request with only a later wake or delegated completion still settles.
• t3_request_attention on a settled thread unsettles it.

After the v0.1.5 upkeep (#135).