feat: add Vertex AI provider support

[ngattusohw]

Summary

Adds Google Cloud Vertex AI as a new LLM provider, enabling authentication via service account credentials and Application Default Credentials (ADC) instead of requiring a GEMINI_API_KEY.

This is useful for teams and production environments that use GCP service accounts, or for anyone running on GCP infrastructure where ADC is automatically available.

New Provider

Prefix: vertex/... (e.g. summarize --model vertex/gemini-2.5-flash-preview https://example.com)

Environment Variables

Variable	Description
GOOGLE_CLOUD_PROJECT	GCP project ID (alias: VERTEX_AI_PROJECT_ID)
GOOGLE_CLOUD_LOCATION	GCP region, e.g. us-central1 (alias: VERTEX_AI_LOCATION)
VERTEX_AI_SERVICE_ACCOUNT_KEY	Inline service account JSON
GOOGLE_APPLICATION_CREDENTIALS	Path to service account JSON file

Also supports GCE metadata server for auth when running on GCP.

Architecture

• New src/llm/providers/vertex.ts with JWT-based service account auth and token caching
• Leverages pi-ai's existing google-vertex API provider for streaming (which uses @google/genai with vertexai: true)
• For document attachments, uses direct REST calls with bearer token auth
• Token is cached with TTL to avoid re-signing JWTs on every call

Auth Priority

1. VERTEX_AI_SERVICE_ACCOUNT_KEY / GOOGLE_SERVICE_ACCOUNT_KEY (inline JSON)
2. GOOGLE_APPLICATION_CREDENTIALS (file path to SA JSON)
3. GCE metadata server (when running on GCP)
4. gcloud auth print-access-token (local dev fallback)

Changes

• Added vertex to LlmProvider type union
• Updated model-id.ts, model-spec.ts, model-auto.ts for vertex provider routing
• Threaded vertexConfig through generate-text.ts, summary-engine.ts, summary-llm.ts, runner.ts, and daemon flow-context.ts
• Updated CLI help text to document vertex env vars and model prefix
• All existing tests pass (56/56 in relevant test files)

Files Changed (12)

• src/llm/providers/vertex.ts — new Vertex AI provider
• src/llm/providers/models.ts — added resolveVertexModel()
• src/llm/model-id.ts — added vertex provider
• src/llm/generate-text.ts — vertex dispatch for complete + stream
• src/model-spec.ts — vertex model spec parsing
• src/model-auto.ts — vertex env key detection for auto mode
• src/run/run-env.ts — vertexConfig + vertexConfigured in env state
• src/run/summary-engine.ts — threaded vertexConfig
• src/run/summary-llm.ts — threaded vertexConfig
• src/run/runner.ts — destructure + pass vertexConfig
• src/run/help.ts — CLI help text
• src/daemon/flow-context.ts — daemon integration