[Feature]: ACP mode — report token usage on session/prompt responses (unstable_end_turn_token_usage)

[panghy]

What would you like to be added?

When running Auggie as an ACP agent (auggie acp / ACP mode as used by Zed and other ACP clients), the session/prompt response currently contains only the stop reason:

{ "stopReason": "end_turn" }

It would be great if Auggie also attached the optional usage field defined by the Agent Client Protocol's unstable_end_turn_token_usage extension — a cumulative-per-session token usage snapshot reported at the end of each turn:

{
  "stopReason": "end_turn",
  "usage": {
    "totalTokens": 12345,
    "inputTokens": 10000,
    "outputTokens": 2000,
    "cachedReadTokens": 300,
    "cachedWriteTokens": 45
  }
}

Notably, the ACP schema bundled in the published npm package (@augmentcode/auggie 0.33.0) already declares usage as an optional field on PromptResponse — the vendored protocol version supports it — but the response never populates it.

Why is this needed?

ACP clients that display per-model/per-session token usage (Zed's agent panel, and other ACP-based hosts that aggregate usage stats) currently show zero tokens for all Auggie sessions, while agents that implement the extension (e.g. the Claude Code ACP adapter, @agentclientprotocol/claude-agent-acp) report usage correctly and get full token dashboards.

Auggie clearly tracks token counts internally (the interactive TUI has /stats and context-usage displays), so the data exists — it just isn't surfaced over ACP. Without it, ACP hosts have no way to attribute token consumption to Auggie turns, and any usage/cost dashboards on the client side are blind for Auggie users.

Possible solution or alternatives

• At the end of each session/prompt turn, attach the cumulative session token totals (input, output, cache read, cache write) to the response, per the unstable_end_turn_token_usage extension shape.
• The semantics expected by clients: the snapshot is cumulative for the session (clients derive per-turn deltas themselves), and the field is simply omitted if counts aren't available.

Additional context

Observed on @augmentcode/auggie 0.33.0 (ACP mode). Related but distinct existing issues: #82 (/stats in --print mode) and #132 (token display in the TUI) — both are about Auggie's own UI/CLI surfaces, whereas this request is about the ACP wire protocol so external clients can consume usage data.

[maxchv]

I need this feature as well