intentd: give delegated Codex ACP sessions task-derived titles #3151

Problem

When Intent delegates work to Codex, the resulting persistent Codex thread appears in the Codex app with the same title: “Delegating Tasks Before delegating, list the tasks to find existing task IDs”.

Intent currently delivers the assembled Codex system prompt through a first-turn prepend because the pinned codex-acp adapter ignores native developer-instruction metadata. Codex title derivation therefore sees the common system-prompt heading before the delegated task.

Proposed direction

Send the existing task-derived Intent agent name out of band as session/new _meta.sessionTitle for Codex sessions, including recreate paths. Keep first-turn prompt ordering and durable resume behavior unchanged.

Upstream adapter support is proposed in agentclientprotocol/codex-acp#338. The local metadata change is backward compatible because older adapters ignore unknown _meta fields; the end-to-end fix requires a released adapter containing that support.

Suppression tradeoff

Codex App Server supports ephemeral threads, but codex-acp does not currently expose that choice through ACP session creation. Intent also relies on loading durable ACP sessions after adapter reap/restart, so making delegated threads ephemeral needs a separate lifecycle/replay design rather than a one-field toggle.

Acceptance

• New delegated Codex threads use the task-derived agent name when the adapter supports sessionTitle.
• The system prompt is not moved or copied into the title.
• Existing session resume/recreate behavior remains reliable.
• Other providers are unchanged.