Emit usage updates from Claude stream events

[Mng-dev-ai]

Summary

• emit ACP usage_update notifications from top-level Claude stream_event message usage
• keep tracking a usage snapshot across message_start and message_delta so partial deltas still produce a complete total
• preserve the existing result-path update as the final exact/cost-bearing usage event

Why

Claude SDK stream events already carry incremental usage on message_delta, but claude-agent-acp currently ignores that path and only emits usage_update from the final result. This makes usage appear stale until the turn ends in ACP clients that expect live context usage updates.

Verification

• built the repo with npm run build
• ran a direct ACP stdio client against the patched local build
• observed usage_update notifications emitted before the final result-path usage update

Patched local smoke-test output included this sequence:

• usage_update used=20973 size=200000
• agent_message_chunk text=TEST_OK
• usage_update used=20989 size=200000
• usage_update used=20989 size=1000000 cost=0.1314875