🎁 feat(session): emit usage_update notification after turns and model

[frostime]

PR: Emit ACP usage updates

Summary

• Emit ACP usage_update notifications with current context usage and cumulative USD cost.
• Refresh usage after session creation or restoration, completed agent turns, and model changes.
• Ignore unavailable or malformed Pi session statistics instead of reporting zero usage.

Rationale

Pi already exposes get_session_stats. Forwarding its context and cost fields lets ACP clients present current usage without parsing agent messages or maintaining a separate estimate.

Implementation notes

• PiAcpSession.refreshUsage() owns Pi statistics normalization and ACP notification emission.
• The refresh counter makes the latest completed request authoritative.
• Existing ACP update serialization is retained.

Validation

• npm run typecheck passed.
• npm run build passed.
• The added usage-update tests passed.
• On the upstream baseline, npm test completed with 89 passing and 2 pre-existing Windows path-separator assertion failures in session-events.test.ts and session-list-and-load.test.ts; neither failure exercises usage updates.

Effect on Zed