Opt-in completion wake for threads started with t3_thread_launch

[kkolodziej7]

Problem

delegate_task children wake their parent when they finish, but they always run in the parent's project and checkout. A thread started with t3_thread_launch (the only way to get its own worktree, or to work in another project) never notifies the thread that launched it.

So an orchestrating agent has to choose: completion wake with a shared checkout, or an isolated worktree with no signal at all. With a cross-project launch there is not even a way to poll, because t3_thread_wait / t3_thread_read are project-scoped (#15125).

In practice the orchestrator ends up running a shell loop that watches for a report file or a PR, and it never learns that the launched run failed (for example a provider turn that dies during startup) unless the user tells it.

Proposal

An opt-in completion notification on t3_thread_launch (and create_threads), e.g. completionWake: true:

• T3 records the launching thread as the watcher of the initial run.
• When that run reaches a terminal state (completed, failed, interrupted, or a preparation failure), T3 delivers one message to the launching thread through the same continuation/mailbox path that async delegate_task uses: steered into an active turn where supported, queued otherwise, replayed after restart, delivered once.
• The message carries the thread id, the terminal status and, on failure, the failure class. Reading the result still goes through t3_thread_read (which needs the launcher-scoped read from [Bug]: Threads launched into another project via t3_thread_launch can't be waited on, read, sent to, or interrupted by the launching thread #15125 for cross-project launches).

This keeps launched threads ordinary top-level threads; it only adds the one-shot signal the launcher currently has to improvise.

Smallest useful version

Wake for the initial run only, default off, no change to permissions or to who may send to the launched thread.

Related

• [Bug]: Threads launched into another project via t3_thread_launch can't be waited on, read, sent to, or interrupted by the launching thread #15125 — launching thread cannot wait/read/send/interrupt a thread it launched into another project
• Feature: Parent is never notified when a delegated child finishes a follow-up turn (V2) #13490 — follow-up turns of delegated children not waking the parent
• Let a thread message threads in other projects, with a grant the user sets #13888 — user-granted cross-project messaging/read/wait