feat(tui): add clear-context plan implementation

[fcoury-oai]

TL;DR

• Adds a second Plan Mode handoff: implement the approved plan after clearing context.
• Keeps the existing same-thread Yes, implement this plan action unchanged.
• Reuses the /clear thread-start path and submits the approved plan as the fresh thread's first prompt.
• Covers the new popup option, event plumbing, initial-message behavior, and disabled states in TUI tests.

Problem

Plan Mode already asks whether to implement an approved plan, but the only affirmative path continues in the same thread. That is useful when the planning conversation itself is still valuable, but it does not support the workflow where exploratory planning context is discarded and implementation starts from the final approved plan as the only model-visible handoff.

Mental model

There are now two implementation choices after a proposed plan. The existing choice, Yes, implement this plan, is unchanged: it switches to Default mode and submits Implement the plan. in the current thread. The new choice, Yes, clear context and implement, treats the proposed plan as a handoff artifact. It clears the UI/session context through the same thread-start source used by /clear, then submits an initial prompt containing the approved plan after the fresh thread is configured.

The important distinction is that the new path is not compaction. The model receives a deliberate implementation prompt built from the approved plan markdown, not a summary of the previous planning transcript. Both implementation choices require the Default collaboration preset to be available, so the popup does not offer a coding handoff when the fresh thread would fall back to another mode.

Non-goals

This change does not alter /clear, /compact, or the existing same-context Plan Mode implementation option. It does not add protocol surface area or app-server schema changes. It also does not carry the previous transcript path or a generated planning summary into the new model context.

Tradeoffs

The fresh-context option relies on the approved plan being sufficiently complete. That matches the Plan Mode contract, but it means vague plans will produce weaker implementation starts than a compacted transcript would. The upside is that rejected ideas, exploratory dead ends, and planning corrections do not leak into the implementation turn.

The current implementation stores the latest proposed plan in ChatWidget rather than deriving it from history cells at selection time. This keeps the popup action simple and deterministic, but it makes the cache lifecycle important: it must be reset when a new task starts so an old plan cannot be submitted later.

Architecture

The TUI stores the most recent completed proposed-plan markdown when a plan item completes. The Plan Mode approval popup uses that cache to enable the fresh-context option and to build a first-turn prompt that instructs the model to implement the approved plan in a fresh context.

Selecting the new option emits a TUI-internal ClearUiAndSubmitUserMessage event. App handles that event by reusing the existing clear flow: clear terminal state, reset app UI state, start a new app-server thread with ThreadStartSource::Clear, and attach a replacement ChatWidget with an initial user message. The existing initial-message suppression in enqueue_primary_thread_session ensures the prompt is submitted only after the new session is configured and any startup replay is rendered.

Observability

The previous thread remains resumable through the existing clear-session summary hint. There is no new telemetry or protocol event for this path, so debugging should start at the TUI event boundary: confirm the popup emitted ClearUiAndSubmitUserMessage, confirm the app-server thread start used ThreadStartSource::Clear, then confirm the fresh widget submitted the initial user message after SessionConfigured.

Tests

The Plan Mode popup snapshots cover the new option and preserve the original option as the first/default action. Unit coverage verifies the original same-context option still emits SubmitUserMessageWithMode, the new option emits ClearUiAndSubmitUserMessage with the approved plan embedded verbatim, and the clear-context option is disabled when Default mode is unavailable or no approved plan exists. The broader codex-tui test package passes with the updated fresh-thread initial-message plumbing.