Adds prompt.go (BuildPrompt + systemPrompt) and pipeline.go (Run, Config,
Result, mergeAll) that wire chunking, LLM calls, parse, merge, index rebuild,
and log append into a single ingestion pipeline. Includes integration tests
covering write, dry-run, and duplicate-path merge scenarios.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
