Add Antigravity usage collector for the agents panel

[candrewlee14]

Summary

Adds the Antigravity usage collector (bin/omarchy-agent-usage-antigravity) to surface Google Antigravity CLI (agy) usage metrics in the omarchy.agents bar widget and panel.

Following up on recent Antigravity default agent integration, this gives Antigravity the same first-class status in the agents drawer as Claude Code, Codex, and Fireworks.

Details

• Metrics Collected:
  • Token metrics: Parses inputTokens, outputTokens (including reasoning/thinking tokens), and cacheReadInputTokens by model (gemini-3.8-flash) from ~/.gemini/antigravity-cli/conversations/*.db.
  • Activity & Prompts: Tracks total and today prompts and sessions from ~/.gemini/antigravity-cli/history.jsonl (with fallback to conversation transcripts in brain/).
  • Tier / Account: Extracts signed-in account from google_accounts.json for the hero tier label.
• Implementation:
  • Single-pass, zero-dependency stream decoder for ModelUsageStats protobufs stored in SQLite gen_metadata.
  • Concurrency safe: reads SQLite databases read-only with PRAGMA query_only = ON.
  • Cached via ~/.cache/omarchy/agent-usage/ with atomic locking. Full disk scan completes in ~20ms.
• UI Assets:
  • Added assets/antigravity.svg and assets/antigravity-light.svg with high-contrast marks matching Omarchy's icon style.
  • Documented Antigravity in shell/plugins/agents/README.md.
• Tests:
  • Added test/shell.d/agent-usage-antigravity-scanner-test.sh testing empty data handling, history.jsonl aggregation, transcript.jsonl fallback, caching, and SQLite protobuf decoding.

Verification

• ./test/shell.d/agent-usage-antigravity-scanner-test.sh: 5/5 tests pass.
• ./test/cli: all suites pass cleanly.
• Tested live on an Omarchy machine running antigravity-cli.