Thread goal updates are flattened to agent message text; expose structured goal state

[Yuyz0112]

Summary

codex emits structured thread-goal events, but codex-acp forwards them only as free-text agent_message_chunks, so ACP clients can't consume the goal's structured state (status/objective/token budget) — they'd have to regex the prose.

Details

codex 0.142 emits:

• thread/goal/updated — goal = { objective, status, tokenBudget }, where status is active | paused | budgetLimited | blocked | usageLimited | complete
• thread/goal/cleared

In src/CodexEventHandler.ts, these map to text-only updates:

• createThreadGoalUpdatedEvent → { sessionUpdate: "agent_message_chunk", content: { type: "text", text: "Goal updated (active): <objective>" } }
• createThreadGoalClearedEvent → { sessionUpdate: "agent_message_chunk", text: "Goal cleared." }

The structured status enum and tokenBudget are dropped. Notably, other thread metadata (name, status, archived) is exposed structurally via createCodexSessionInfoUpdate → session_info_update with _meta.codex. Goal state is the odd one out.

Impact

A client can't reliably detect goal start (active) / end (complete) / clear, or surface the token budget, without parsing localized/format-fragile text.

Suggested fix

Route goal updates through the same structured channel already used for thread status/archived/name, e.g.:

session_info_update  with  _meta.codex.goal = { objective, status, tokenBudget }

(and a cleared signal on thread/goal/cleared). The human-readable agent_message_chunk can stay for display; the ask is to also expose the structured state so clients can react to the goal lifecycle programmatically.