Add architecture decision records (ADRs)

[rolandpg]

Summary

Document key architecture decisions as ADRs (Architecture Decision Records).

Context

ADRs explain why things are built the way they are. They help contributors and users understand trade-offs without reading through commit history or Slack threads.

• Directory to create: docs/adr/
• Format: Michael Nygard's ADR template (Status, Context, Decision, Consequences)

Acceptance Criteria

• Create docs/adr/ directory with a README.md explaining the ADR format
• Write at least 4 ADRs:
  • ADR-001: JSONL for community storage — Why not SQLite from day one? (simplicity, append-only, human-readable, zero config)
  • ADR-002: Blended retrieval (vector + graph) — Why not vector-only? (graph adds entity relationships, intent routing weights both)
  • ADR-003: In-process LLM (llama-cpp-python) — Why not API-only? (offline-first, no API keys, security-conscious users)
  • ADR-004: STIX 2.1 entity types — Why not custom types? (interoperability, industry standard, OpenCTI compatibility)
• Each ADR has: Title, Date, Status (Accepted), Context (2-3 sentences), Decision (1-2 sentences), Consequences (pros and cons)
• Each ADR is 100-300 words (concise, not essays)
• ADRs are numbered sequentially and listed in docs/adr/README.md

References

• ADR format: https://adr.github.io/
• Example: https://github.com/joelparkerhenderson/architecture-decision-record