Add Google Antigravity tracing integration

[Stephen Belanger (Qard)]

Summary

• add a thin, fail-open Google Antigravity hook plugin for PreInvocation, PostInvocation, PostToolUse, and Stop
• add a daemon translator that combines lifecycle hooks with bounded transcript_full.jsonl records to produce session, turn, LLM, and tool spans
• preserve deterministic replay and correct multi-turn resume behavior when Antigravity resets its process-local invocation counter
• allow resumed sessions to extend an already-ended root span and route explicit project IDs without registering a project name
• add bt trace setup antigravity and bt trace setup antigravity --disable
• add plugin validation plus translator, setup, and sink regression coverage

Why

Antigravity's hook payloads provide model/tool lifecycle boundaries, while its full transcript contains the message and tool content needed for useful traces. Combining both sources makes the core integration feasible without changing Antigravity's permission behavior.

PreToolUse is intentionally omitted because its response participates in authorization. The adapter remains credential-free and fails open; authentication and destination routing stay in the bt host.

Setup behavior

bt trace setup antigravity stages the bundled adapter, installs it through Antigravity's agy plugin install, registers the direct global hooks needed by the current Antigravity runtime, and persists the selected non-secret Braintrust route. Repeated setup replaces only the managed entry and preserves unrelated hooks.

bt trace setup antigravity --disable does not require Braintrust auth or destination selection. It uninstalls the managed plugin, removes only the Braintrust hook entry, and marks persistent tracing disabled while preserving the saved route and unrelated settings.

Live validation

A real Antigravity 1.1.12 session was delivered automatically through the daemon to the existing coding-agents project:

https://www.braintrust.dev/app/Braintrust%20SDKs/p/c77f5203-f2ef-4ed6-8f45-049d082bc5f0/logs?r=47982939-b057-5e9d-a4c1-b5a60425f0c3&s=47982939-b057-5e9d-a4c1-b5a60425f0c3

The resulting trace contains:

• Turn 1 with an LLM tool request, successful list_dir span, and final LLM response
• Turn 2 with its own correctly parented LLM span and response
• a session root whose duration extends through Turn 2

The setup command was also exercised through a locally built real bt host and the installed agy 1.1.12 CLI in an isolated Antigravity home. Initial setup, repeated setup, JSON output, route persistence, plugin discovery, and disable all passed without touching the user's active Antigravity configuration.

Validation

• make validate-antigravity
• cargo test --manifest-path bt-daemon/Cargo.toml --all-features
• cargo clippy --manifest-path bt-daemon/Cargo.toml --all-targets --all-features -- -D warnings
• cargo fmt --manifest-path bt-daemon/Cargo.toml -- --check
• real bt trace setup antigravity / repeat / --disable lifecycle against agy 1.1.12 in an isolated home
• git diff --check

The all-feature test run passed; four unrelated real-agent tests remained intentionally ignored because they require their respective installed CLIs or built extensions.

Current scope

This establishes the core hooks, translator, and persistent setup flow. Managed-run injection, historical import/attach, Windows support, and production distribution remain follow-up work. After this repository change merges, the released bt CLI still needs its bt-daemon revision bumped to expose the new setup subcommand.