feat(openab-agent): add Google Gemini provider (API key)

[Joseph19820124]

What

Adds a Google Gemini provider to openab-agent, mirroring the existing AnthropicProvider API-key pattern.

Review scope: this PR's base is the skills branch (a verbatim copy of upstream PR openabdev#955 "feat(openab-agent): add skill support"), so the diff here is only the Gemini commit — the skills code is excluded from review noise. The full branch-vs-main view (skills + Gemini) is at the compare link below.

The Gemini provider

GeminiProvider in openab-agent/src/llm.rs, modeled exactly on AnthropicProvider:

• Credentials: from_env() reads GEMINI_API_KEY, falling back to GOOGLE_API_KEY — same shape as AnthropicProvider reading ANTHROPIC_API_KEY. No OAuth.
• Format mapping: translates the internal Anthropic-shaped Message/ContentBlock model to Gemini's generateContent:
  • role assistant → model
  • tool_use → functionCall
  • tool_result → functionResponse
• Two impedance mismatches handled:
  • Gemini keys a functionResponse by the function name, not an id → the name is recovered from the originating tool_use block (gemini_tool_name).
  • Gemini functionCalls carry no id → a stable id call_<name>_<idx> is synthesized so the agent loop can pair the eventual tool_result back to the call.
• Retries: 429 / 503 with exponential backoff, like the other providers.
• Wiring (acp.rs): explicit OPENAB_AGENT_PROVIDER=gemini/google, plus the auto-detect chain Anthropic key → Gemini key → Codex OAuth. Error message updated to mention GEMINI_API_KEY.
• Config: OPENAB_AGENT_MODEL (default gemini-2.0-flash), OPENAB_AGENT_GEMINI_BASE_URL.

Tests & checks

• cargo test — 30 passed / 0 failed (5 new Gemini tests: role mapping, tool_result keyed-by-name, error field, parse text/functionCall, error envelope).
• cargo build, cargo fmt --check, cargo clippy — clean (Gemini code: 0 warnings).
• docs/native-agent.md updated (env-var table + auth section).

Notes

• Default model gemini-2.0-flash is deliberately conservative; override via OPENAB_AGENT_MODEL.
• Opened inside this fork only — upstream openabdev/openab is untouched.
• Full branch (skills + Gemini) vs fork main: main...feat/gemini-provider-and-skills

🤖 Generated with Claude Code

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[Joseph19820124]

Superseded by two independent PRs against upstream-main: #5 (Agent Skills, replicating upstream openabdev#955) and #6 (Google Gemini provider). This stacked PR (Gemini on top of skills) is no longer needed.