Proposal: add Google Antigravity and Gemini models to Usage reporting

[nickwarters]

Context

With Google Antigravity recently added as an ACP provider (#9348), turns run on Gemini models (e.g., gemini-3.8-flash, gemini-2.5-pro) currently do not appear in T3 Code's Usage graphs and breakdowns.

Following the architecture established for Claude Code, Codex, and Grok Build (#8358), UsageService aggregates usage by scanning local session files on disk so that usage metrics stay complete across connected environments and for sessions driven outside T3 Code.

Technical Findings

Antigravity persists turn history and exact token metrics locally in SQLite database files (<conversationId>.db):

• T3-managed profiles: <stateDir>/providers/antigravity/<instanceHash>/antigravity-acp/conversations/*.db
• Standalone CLI: ~/.gemini/antigravity-cli/conversations/*.db

Inside each conversation database:

• The gen_metadata table stores generation turns with a Protocol Buffers payload (data) per turn.
• The record contains:
  • Model string: e.g., gemini-3.8-flash, gemini-2.5-pro (field 1.19).
  • Token usage: Uncached input tokens, candidate/output tokens, reasoning/thought tokens, and cached tokens (field 1.4).
  • Timestamp: Generation timestamp (field 1.9.4.1).
  • Request ID / Dedupe Key: e.g., <conversationId>-<turn> (field 1.20).

Because Node.js includes node:sqlite (DatabaseSync) in read-only mode, and the wire format for these fields is straightforward to extract in pure TypeScript without external dependencies, no new dependencies are required. Unchanged databases are memoized by (size, mtime) in ScanCache just like existing JSONL transcripts.

Additionally:

• LiteLLM's rate table (usagePricing.ts) already catalogs 180+ Gemini models (with unlisted newer models falling back to "unpriced" gracefully while keeping token counts accurate).
• Antigravity icons and assets already exist across both web (AntigravityIcon) and mobile (assets/antigravity.png).

Proposed Scope

1. Contracts (packages/contracts/src/usage.ts):
   • Bump USAGE_CONTRACT_VERSION to 6 (retaining USAGE_MERGE_COMPATIBLE_SINCE so v4/v5 servers continue to merge).
   • Add "antigravity" to UsageProviderKind.
2. Server (apps/server/src/usage/):
   • In UsageService.resolveTranscriptDirs, discover managed and standalone Antigravity conversation directories.
   • Extract generation records from SQLite in read-only mode into standard UsageRecord objects.
3. Web & Mobile Presentation:
   • Add antigravity presentation configuration to apps/web and apps/mobile (PROVIDER_ORDER, PROVIDER_PRESENTATION, brand colors).

Question for Maintainers

Would maintainers be open to a PR implementing Antigravity usage reporting along these lines? If so, I have tested the SQLite/protobuf extraction against active sessions and can submit a focused PR with unit tests.