Add optional `intent` parameter that steers query expansion, reranking,
chunk selection, and snippet extraction without searching on its own.
When a query like "performance" is ambiguous (web-perf vs team health vs
fitness), intent provides background context that disambiguates results
across all pipeline stages:
- expandQuery: includes intent in LLM prompt ("Query intent: {intent}")
- rerank: prepends intent to rerank query for Qwen3-Reranker
- chunk selection: intent terms scored at 0.5x weight vs query terms
- snippet extraction: intent terms scored at 0.3x weight
- strong-signal bypass: disabled when intent provided
Available via CLI (--intent flag or intent: line in query documents),
MCP (intent field on query tool), and programmatic API.
Adapted from PR #180 (thanks @vyalamar).
