Runtime: add a native Google Gemini provider (`provider: gemini`)

[initializ-mk]

Summary

forge's runtime supports three model providers today — anthropic, openai, and bedrock. Add Google Gemini as a first-class native provider (provider: gemini), covering both Vertex AI and the Gemini Developer API.

Model this on the native AWS Bedrock Converse provider (#205), not on the openai path.

Why native (not the OpenAI-compat shim)

Gemini's native API is a distinct wire format — generateContent / streamGenerateContent with contents[].parts[], roles user/model, systemInstruction, tools:[{functionDeclarations}], and generationConfig — not chat-completions and not Responses. Google does expose an OpenAI-compatible chat/completions endpoint, but routing through it drops Gemini-native features (Google Search grounding, thinking config, safety settings). So this should be a native provider that speaks generateContent, exactly as bedrock speaks Converse.

Scope

• Provider impl — forge-core/llm/providers/gemini.go implementing the provider interface (streaming via streamGenerateContent), translating forge's message/tool model to/from Gemini's contents/parts (roles user/model), systemInstruction, functionDeclarations, and generationConfig. Mirror bedrock.go / bedrock_eventstream.go.
• Factory + client — register gemini in forge-core/llm/providers/factory.go; wire in forge-core/llm/client.go.
• Config + validation — provider: gemini in forge-core/types/config.go; in forge-core/validate/forge_config.go require the Vertex fields (project + location/region) when in Vertex mode, or an API key for the Developer API — analogous to bedrock requiring aws_region.
• Auth — Vertex AI via Google ADC / service account (the google-auth chain); Gemini Developer API via API key. Support a base_url override for routing (the ANTHROPIC_BASE_URL analog).
• forge init — offer Google Gemini as a first-class provider in the TUI and non-interactively (--model-provider gemini + the Vertex project/region or API key), with a default Gemini model list.
• Tests — mirror bedrock_test.go / responses_test.go (wire-format translation, streaming, tool calls, validation).

Acceptance

• provider: gemini runs an agent against a Gemini model (Vertex AI and Developer API), with streaming
• Tool calls (functionDeclarations) round-trip
• forge validate enforces the required Gemini/Vertex config
• forge init offers Gemini as a first-class provider
• Docs + CHANGELOG entry (as with the Bedrock provider, Phase 2.5: AWS Bedrock URL + body rewriting for native InvokeModel passthrough #205)