fix: suppress default context usage updates

[jsgrrchg]

Summary

Suppress live usage_update notifications while the session context window is still the generic 200000 fallback placeholder.

Details

This keeps accumulating streamed usage internally, but avoids publishing a misleading live size value until the context window is known from either a model-name heuristic or authoritative result.modelUsage.

Final result usage updates are unchanged, so clients still receive the correct usage, cost, and context window once modelUsage.contextWindow is available.

Fixes #596

Testing

• npm test -- --run src/tests/acp-agent.test.ts -t "usage_update computation"
• npm run build
• npm run test:run
• npx prettier --check src/acp-agent.ts src/tests/acp-agent.test.ts src/tests/session-config-options.test.ts