Start an Architecture Decision Records (ADR) log

[Amayyas]

What

Add docs/adr/ with a first ADR or two documenting decisions already made implicitly (e.g. "why keyword-based router instead of LLM-based in the MVP", "why context slicing instead of full history replay"). Several of the architecture issues above (LLM router, explicit handoff, context compression) should get their own ADR before implementation starts.

Acceptance criteria

• docs/adr/ structure added with a chosen ADR template
• At least one retroactive ADR written documenting an MVP-era decision
• Linked from README/CONTRIBUTING