[ACP] Emit `usage_update` in `--acp` mode (context window + AI credits) for parity with the interactive statusline

Describe the feature or problem you'd like to solve

copilot --acp never emits the ACP usage_update session update, so ACP clients (Zed, etc.) can't show a context-window or AI-credit usage indicator for Copilot — even though the CLI already computes this data for /context, /usage, and the statusline.

Proposed solution

Summary

When Copilot CLI runs in ACP mode (copilot --acp), it never sends the ACP
session/update notification with sessionUpdate: "usage_update". As a result,
ACP clients (Zed and other editors/tools integrating over ACP) cannot show a
context‑window usage indicator or AI‑credit usage for Copilot — even though the
CLI already computes this data and surfaces it in interactive mode.

The gap: interactive mode has it, ACP mode doesn't

In interactive mode the data is already there:

• The experimental statusLine.command receives JSON with
  context_window.current_context_tokens, context_window.displayed_context_limit,
  context_window.current_context_used_percentage, etc.
• /context shows the context‑window breakdown.
• /usage shows AI credits used in the current session.

But in --acp mode none of it is forwarded — the client only ever receives
agent_message_chunk, tool_call, available_commands_update, etc., and never a
usage_update (even though the type is part of the ACP schema the CLI already speaks).

Why it matters

• ACP standardized session usage reporting (Session Context Size & Cost RFD).
  usage_update carries { used, size } (+ optional cost) and powers Zed's
  context‑window indicator.
• Other ACP agents (e.g. Claude Code via claude-agent-acp) emit usage_update, so
  their users get a live context indicator in Zed. Copilot users don't — Zed falls
  back to inaccurate estimation (see zed-industries/zed#44909, where Copilot models
  show a hardcoded 128k limit that doesn't match the real limit).
• For long agentic sessions, knowing how close you are to compaction — and how many
  credits you've spent — is valuable.

Request

In --acp mode, emit session/update with sessionUpdate: "usage_update":

• After session/new, session/load, session/resume, and after each prompt turn.
• Populate the standard fields: used (current context tokens), size (context
  window size), and cost (session cost) when available.
• For AI credits (used / remaining) — which have no standard ACP field — attach them
  via the update's _meta (already part of the usage_update schema), e.g.
  _meta: { ai_credits_used, ai_credits_remaining }.

This brings ACP mode to parity with the interactive statusline / /context /
/usage, with no new data to compute — just forwarding what already exists over the
protocol the CLI already implements.

References

• ACP usage_update (Session Context Size & Cost RFD)
• Zed context indicator; zed-industries/zed#44909 (Copilot token limits estimated / incorrect)
• Related CLI requests: #2052 (persistent token/context indicator), #1953 (always‑visible context status)

Example prompts or workflows

No response

Additional context

No response

Start at the --acp entry point and trace session/new, session/load, session/resume, and prompt-turn handling. Compare the existing /context, /usage, and statusline data with the ACP session/update schema, then verify that usage_update reports used, size, cost, and the requested AI-credit metadata after each lifecycle event.