Source-Aware PRD Ingestion — aggregate goals from adjacent docs

[DevVig]

Summary
Augment the Assess phase to discover and ingest adjacent domain docs (e.g., value proposition, roadmap, market validation) to seed PRD sections with real content. Offer a preview and require confirmation before writing.

Why

• Many repos already contain rich business docs (like Cyon). We should leverage them instead of generating placeholders.

Scope

• Discovery + extraction pipeline with heuristics; safe preview mode

Tasks

• Implement a2a/analyst/prd_aggregator.py:
  • Discover files by name/keywords: *VALUE*OFFERING*, *ROADMAP*, market*validation*, vision*, mission*, strategy*
  • Extract candidate content for sections (Goals, Personas, NFRs, Analytics, Rollout)
  • Summarize into structured JSON and a preview markdown
• Add a2dev prd-ingest --preview CLI to show diff and let user accept
• Wire ingest step into @analyst assess prepared as an optional enhancement
• Cache parsed results; record sources in docs/timeline/

Acceptance Criteria

• On Cyon, ingest pulls concrete goals/aims from CYON_VALUE_OFFERING_COMPREHENSIVE.md and roadmap files
• Preview shows changes; accept writes to docs/PRD.md with citations in comments
• Deterministic runs with temp=0; no hallucinated content

References

• Cyon: CYON_VALUE_OFFERING_COMPREHENSIVE.md, CYON_FEATURE_ROADMAP_2025.md