feat: add the Antigravity agent driver (@agentry/antigravity)

[dortort]

Summary

Adds @agentry/antigravity, a driver that drives Google's Antigravity CLI (agy) headlessly via agy -p --output-format stream-json and normalizes its native stream into the shared Agentry AgentEvent model — the same contract @agentry/claude implements. Wires it into the CLI so agentry test/record can target Antigravity with use: { agent: 'antigravity' }.

Antigravity supersedes the Gemini CLI for personal use; its agy CLI is a genuine headless, structured-output agent runner, so it earns a first-class driver.

How it works

The event schema was captured live from agy 1.1.11 (a fresh feasibility spike). Antigravity's stream is discriminated on event (not type). parseAntigravityEvents is a pure whole-stream mapper:

Native (agy --output-format stream-json)	AgentEvent
init{init:{cwd,tools}}	run.start
step_update{step_type:'tool', state:'ACTIVE'/'DONE'}	tool_use/tool_result
step_update{step_type:'agent_response', text_delta}	message (coalesced by step_index)
result{status,usage}	usage + run.end

Key details: agent_response text streams as deltas, coalesced per step_index into one message. Usage and run.end come from the authoritative terminal result event (input_tokens/output_tokens/cache_read_tokens). Spawns with stdin ignored.

Capabilities

{ structuredStream: true, llmInterception: 'none',
  mcpTransports: [], toolPermissionControl: true, nativeBudgetControl: false }

Model ids are agy's own pinned names, e.g. claude-sonnet-4-6 or gemini-3.5-flash-medium.

Known limitations (MVP)

• Sandbox fs-diff is best-effort. agy writes to its own project/scratch dir by default rather than the process cwd; buildArgs passes --add-dir <sandbox>, but the agent may still use its scratch dir, so file side-effect assertions on the sandbox may miss writes. Event-stream normalization (tools, messages, usage) is fully faithful.
• LLM interception is 'none' (routed through Antigravity's backend; not interceptable via env base-URL). Transcript record/replay works; wire cassettes do not apply.
• MCP transports unverified, declared [] rather than overclaiming.
• allowedTools/disallowedTools/mcpConfig/maxBudgetUSD/appendSystemPrompt are ignored in MVP.

Testing

• pnpm typecheck clean; new-package tests green (parser incl. per-step delta coalescing + buildArgs); full suite 93 tests, 0 failures, no regressions.
• Parser fixtures are the live-captured agy 1.1.11 stream.
• agentry doctor probes the agy CLI (found v1.1.11) and prints the driver's capabilities.

Merge notes

• Branched off main. Additively touches three files the sibling Codex/Gemini PRs also touch — the selectDriver switch in packages/cli/src/commands/run.ts, doctor.ts, and the tsconfig.json/vitest.config.ts aliases. Whichever PR merges later needs a trivial union-merge.
• No changeset included: the changesets system is being introduced separately on chore/release-workflow. When that lands, add @agentry/antigravity to the fixed-version group and a changeset entry.

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