feat(app-server): filter threads by parent

[btraut-openai]

Why

Clients that display or coordinate spawned subagents need an authoritative snapshot of a thread's immediate spawned children when they connect to app-server or recover after missing live events. thread/list cannot query by parent, so clients must otherwise scan unrelated threads or reconstruct relationships from rollout history and transient events.

The direct spawn relationship already exists in persisted thread_spawn_edges state. Review and Guardian threads do not participate in that lifecycle and are intentionally outside this filter's scope.

What changed

This adds an experimental parentThreadId filter to thread/list. Parent-filtered requests return direct spawned children from persisted state while preserving the existing response shape, explicit filters, sorting, and timestamp-only cursor behavior. The lookup does not read rollout transcripts or recursively return descendants.

Supersedes #25112 with the narrower thread/list filter approach.

How it works

1. An experimental client passes a valid thread ID as parentThreadId.
2. App-server routes the list through the existing thread-store and state-database boundaries.
3. SQLite selects threads whose IDs have a direct persisted spawn edge from that parent.
4. Omitted provider and source filters include all values; explicit filters keep ordinary thread/list semantics.
5. Grandchildren, Review threads, and Guardian threads are excluded.

Verification

State (144 tests), rollout (69 tests), and focused app-server thread-list (31 tests) suites passed. Scoped Clippy checks and repository formatting also passed. Coverage includes direct spawned children, omitted grandchildren, pagination, malformed IDs, mixed source kinds, explicit filters, and operation without rollout files.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[btraut-openai]

Codex: This is intentionally a narrow accommodation in the existing rollout/state adapter. Conceptually, querying indexed thread metadata and validating rollout-file existence would be separate layers; parent-filtered listing only needs the first. Extracting that boundary here would broaden the PR and risk existing thread/list behavior, so this branch returns authoritative persisted relationships before rollout validation while leaving the ordinary path unchanged.

[chatgpt-codex-connector]

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[jif-oai]

This won't be backward compatible but lgtm after my comments
Please check with DDR as well, he did some work on sub-second ordering / reflection