Child threads: hidden from the sidebar, opened and messaged from the parent's Agents panel

[lukemaj]

Outcome

Threads started by another thread (Model Router jobs, send) are real T3 threads that stay out of the sidebar, appear in the parent thread's Agents panel, open on click, and can be messaged there. This is the production version of spike #3 / draft PR #4, and the fork capability Model Router's send (toolboxmd/model-router#92) and T3-thread jobs build on.

Acceptance criteria

• A real parent link on threads: prefer an optional parentThreadId on thread.create carried to the thread shell over the spike's sub.<parent>.<suffix> id convention; if the contract change is too invasive for a thin fork, keep the id convention and say why.
• Child threads are hidden from the sidebar (no extra section).
• The parent's Agents panel lists its child threads with status, and a row opens the child thread; the normal composer messages it.
• The threads MCP toolkit from the spike (create, message, read) ships, with its safety rule: no message to a child that has not finished starting.
• Upstream edits minimal and listed with files and lines in the PR body; new code in new files.
• Upstream checks pass: vp run -r typecheck, vp lint, vp run -r test, vp fmt --check.
• Proven live on an isolated T3 instance (never the user's instance on port 3773 or ~/.t3/userdata) with at least a Codex and an OpenCode child.

Non-goals

A back-link UI from child to parent (Model Router puts the parent link in the child's first message), mobile, OpenBot.

Blockers

None.

Objective contribution

Second completion criterion of the fork Objective: Model Router jobs run as T3 threads.

[lukemaj]

Submitted to Model Router as job t3code-8-1101 (source runner at model-router e837872, state dir ~/.local/share/durable-runner-planner-f1213, planner: Claude thread f1213d48). One PR will follow; not merged.

[lukemaj]

Production child threads are ready for review in #10 (branch feat/8-child-threads, commit 8e53f43).

What shipped: threads spawned by another thread are real T3 threads with a sub.. id (kept over a real parentThreadId — the contract, projector, persistence and migration change was too invasive for this fork; reasoned in the PR body), hidden from both sidebars, listed with status in the parent's Agents panel, openable there and messageable via the composer, plus the threads MCP toolkit (spawn, message, read, list) with the still-starting guard.

Proof: install/typecheck/lint/fmt all exit 0; repo tests pass except one pre-existing unrelated failure (scripts/build-desktop-artifact.test.ts Windows-payload case, deterministic, zero overlap). New unit tests cover the status vocabulary, effort routing, delivery rule and id helpers. Live on an isolated instance (/tmp home, port 14768): a Codex child and an OpenCode child each ran to idle with exact-match replies, the starting guard fired with its exact retryable error, failed status surfaced with the provider error, list/read tracked all three children, and the composer path replied COMPOSER_OK. Details and upstream-edit line list are in the PR body.