fix(curate): improve Vertex AI reliability for large curation extractions

[mpeter]

Fixes three bugs that cause dewey curate to fail consistently on real-world vaults with moderate content density.

Bugs Fixed

#76 — MaxTokens hardcoded at 4096 in VertexSynthesizer

Real curation extractions produce 4K-16K output tokens. Claude truncates the JSON array at the 4096 token limit, causing ParseExtractionResponse to fail with unexpected end of JSON input.

// Before
MaxTokens: 4096,

// After
MaxTokens: 16000,

#77 — HTTP timeout too short for Vertex AI

Vertex AI with Claude Sonnet takes 120-260s for 30K-40K token prompts. The 120s HTTP client timeout cuts off large extractions mid-stream. Vertex has fundamentally different latency characteristics than local Ollama inference.

// Before
Timeout: 120 * time.Second,

// After
Timeout: 300 * time.Second,

#78 — quality_flags string type crashes ParseExtractionResponse

LLMs non-deterministically output "quality_flags": "none" (string) instead of "quality_flags": [] (array). Strict json.Unmarshal into []KnowledgeFile rejects the entire response for a type mismatch on a single field.

Changed to parse []json.RawMessage first, then unmarshal each item individually. Items that fail strict unmarshal fall back to a permissive struct that accepts any JSON type for quality_flags and discards non-array values. Valid items are preserved; unparseable items are logged and skipped.

Additional: prompt conciseness

The extraction prompt now requests concise output (3-8 items per document maximum, short excerpts ≤20 words, content ≤30 words per item). This reduces output token consumption, making truncation less likely even near the max_tokens limit, and produces more scannable curated files.

Testing

Tested against 3 knowledge stores with varying content density (9, 10, and 42 documents respectively). All previously failed with one or more of the three bugs above and now produce knowledge files successfully.

Closes #76, #77, #78