feat: add the Gemini agent driver (@agentry/gemini)

[dortort]

Summary

Adds @agentry/gemini, a driver that drives the Gemini CLI headlessly via gemini -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 Gemini with use: { agent: 'gemini' }.

Delivers part of the Phase 5 "cross-agent adapters" roadmap item (spike-gated).

How it works

The event schema was captured live from gemini-cli 0.54.4 (the Phase 0.12 feasibility spike) and cross-checked against the CLI's bundled StreamJsonFormatter. parseGeminiEvents is a pure whole-stream mapper:

Native (--output-format stream-json)	AgentEvent
init	run.start
message{role,content,delta?}	message
tool_use{tool_name,tool_id,parameters}	tool_use
tool_result{tool_id,status,output}	tool_result
error{severity,message}	error
result{status,stats}	usage + run.end

Key detail: Gemini streams assistant text as multiple {role:"assistant", delta:true} chunks. The parser coalesces consecutive assistant deltas into a single message (so turns/lastMessage stay correct) — this is why it's whole-stream rather than per-event. Usage and run.end come from the terminal result event. Spawns with stdin ignored.

Capabilities

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

Known limitations (MVP)

• buildArgs passes --skip-trust — gemini 0.5x refuses untrusted workspaces in headless mode.
• LLM interception via base-URL is unproven, so declared 'none' and not wired to Agentry's proxy. Transcript record/replay works; wire cassettes do not apply.
• disallowedTools/mcpConfig/maxBudgetUSD/appendSystemPrompt have no direct headless equivalent and are ignored.

Testing

• pnpm typecheck clean; new-package tests green (parser incl. delta coalescing + buildArgs); full suite 92 tests, 0 failures, no regressions.
• Parser fixtures are the live-captured gemini-cli 0.54.4 stream.
• agentry doctor probes the gemini CLI and prints the driver's capabilities.

Merge notes

• Branched off main. Additively touches three files the sibling Codex/Antigravity 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/gemini to the fixed-version group and a changeset entry.
• Heads-up (environment, not this PR): upgrading the local Gemini CLI to 0.54.x drops support for the free "Code Assist for individuals" OAuth tier (IneligibleTierError); an API key (GEMINI_API_KEY) works. Relevant for anyone running live/record mode.

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