refactor(goal)!: stop emitting the active_goal stream event - #12181

What this PR does

--output-format stream-json no longer emits the active_goal stream event. It was a projection of goal_state into the pre-#7895 card shape, emitted after every goal_state when partial messages were on. With it go the second projection core kept to decide when to emit it (projectActiveGoal, sameActiveGoalProjection and their bookkeeping in client.ts), LlmEventType.ActiveGoal, ServerLlmActiveGoalEvent and its deprecated ServerGemini alias, the ActiveGoal wire type, the headless ActiveGoalStreamEvent, and the two break cases the TUIs kept to ignore it. goal_state is emitted exactly as before, with or without --include-partial-messages.

Why it's needed

Step 5-b-1 of #12179. docs/users/features/headless.md has documented active_goal as a compatibility projection with goal_state authoritative since #7895 (2026-07-29); neither TUI reads it (use-llm-stream.ts, event-adapter.ts both ignore it); nothing else in this repository consumes it. It costs one extra stream event per Goal transition and ~60 lines of projection kept in step with the canonical state.

Reviewer Test Plan

How to verify

• StreamJsonOutputAdapter.test.ts: a GoalState event with partial messages on produces goal_state as the only Goal stream event; without partial messages it still does. The cases that asserted active_goal followed it are gone.
• nonInteractiveCli.test.ts: /goal in headless stream-json mode with partial messages emits ['goal_state'].
• client-goal.test.ts: ordering assertions now anchor on the goal_state events themselves (initial state, pause, turn_finished) instead of the projection that followed them.
• event-adapter.test.ts: an unknown event type maps to nothing (the former active_goal case).
• Type-level: LlmEventType.ActiveGoal and the ActiveGoal type no longer exist, so any remaining consumer fails to compile.

Evidence (Before & After)

A real headless run on the bundled branch, -p "/goal set …" --output-format stream-json --include-partial-messages, tallied by event.type:

178 content_block_delta
 21 content_block_start
 21 content_block_stop
  3 goal_state
  4 message_start
  4 message_stop

No active_goal entries; on main the same run emits one after each goal_state. Local: core client, client-goal, goal-legacy-projection, turn: 538 passed; cli StreamJsonOutputAdapter (both files), nonInteractiveCli, event-adapter, use-llm-stream: 570 passed; tsc --noEmit clean for core and cli; core builds; the JS bundle builds; ESLint and Prettier clean on every changed file.

Tested on

Environment (optional)

Unit tests plus one headless stream-json run against the bundled dist/cli.js.

Risk & Scope

• Main risk or tradeoff: automation that still filtered stream-json for active_goal stops seeing it. The doc line that introduced it as a compatibility projection now says it is no longer emitted and to read goal_state; the ! in the title puts it in the release notes' breaking-changes list.
• Not validated / out of scope: the ACP _meta.goalStatus projection and goal-legacy-projection.ts itself, which stay for the Web Shell's history card until 5-b-2 (Goal: retire the legacy card projection (ladder step 5-b) #12179).
• Breaking changes / migration notes: LlmEventType.ActiveGoal, ServerLlmActiveGoalEvent, ServerGeminiActiveGoalEvent and ActiveGoal leave the core exports; ActiveGoalStreamEvent leaves the cli's headless types. Consumers read goal_state / ServerLlmGoalStateEvent.

Linked Issues

Part of #12179 (5-b-1). Ladder: #12053.