docs(adr): 0001 — Adapter contract (v1)

[EmersonBraun]

Summary

First ADR of Phase 0 — formalizes the Adapter contract that has been implicit in packages/core/src/types/adapter.ts.

What's included

• docs/architecture/adrs/0001-adapter-contract.md — the contract itself: 4 types + 10 invariants (A1-A10), stability tier (stable), rationale, consequences, alternatives considered, and open questions.
• docs/architecture/adrs/README.md — ADR process (what, when, format, index).

Why this matters

Before Fase 3 adds 10+ new adapters (Mistral, Cohere, Groq, Bedrock, xAI/Grok, local models via Ollama/llama.cpp, etc.), we need a verifiable contract. Today "plug-and-play" is aspirational. After this ADR:

• Writing a new adapter becomes a closed problem: satisfy 10 invariants, ship.
• A future AdapterContractSuite can validate any adapter mechanically.
• Ensemble, router, and fallback-chain adapters ([F2] #32 — Adapter router (auto-selection) #145 [F2] #33 — Adapter ensemble (N models + aggregation) #146 [F2] #34 — Declarative adapter fallback chain #147) compose cleanly because every leaf adapter is interchangeable.
• Deterministic replay ([F2] #21 — Deterministic replay engine #134) has a stable foundation.

Key invariants (the important ones)

• A1 Pure factory — createSource does no work until stream() is called (enables retry/replay)
• A3 Terminal chunk — every stream ends with done, error, or caller-initiated abort (no silent endings)
• A4 Chunk purity — text chunks are meaningful in isolation (unlocks speculative execution)
• A7 No input mutation — required for replay and ensemble
• A9 Error-as-chunk — failures do not throw from stream(), they emit a chunk

Trade-offs made explicit

• Partial-args tool calling deferred to a v2 ADR — v1 requires complete args in a single chunk
• metadata is untyped (Record<string, unknown>) — provider-specific data lives there, consumers must not depend on its shape
• Cost/token counting intentionally NOT in the contract — belongs in observability layer

Follow-ups (tracked in #214)

• ADR 0002 — Tool contract
• ADR 0003 — Memory contract
• ADR 0004 — Retriever contract
• ADR 0005 — Skill contract
• ADR 0006 — Runtime contract

Test plan

• Doc renders on GitHub
• ADR index updated
• No code changes — pure documentation
• Reviewer: agree A1-A10 match current adapters in packages/adapters?
• Reviewer: any invariant missing or too strong?

Refs #214 #211