docs: CONVENTIONS.md per package [closes #232]

[EmersonBraun]

Summary

One focused CONVENTIONS.md per package explaining:

• Scope — what belongs here
• What does NOT belong here — routing to sibling packages
• How to add a new X — step-by-step for adapters, tools, skills, memory backends, components, observers, etc.
• Testing conventions specific to that package
• Common pitfalls as a lookup table
• Review checklist reviewers run when approving PRs

Closes #232.

Highlights per package

• core — strictest rules. Zero deps, <10KB, no any, named exports only. Explicit list of what triggers an ADR.
• adapters — step-by-step for a new chat adapter + embedder, naming rules, contract-test pointer (10 invariants).
• react — headless components, data-ak-* attributes for styling hooks, accessibility requirements.
• ink — documents the ink@7 / ink-testing-library@4 test harness workaround so new contributors don't rediscover it.
• cli — --help fits one screen, stdout vs stderr discipline, exit code conventions.
• runtime — enforces RT1–RT14 as the review gate. Any PR touching the loop must argue why each invariant still holds.
• tools — JSON Schema 7 (not Zod), requiresConfirmation: true for destructive ops, snake_case naming.
• skills — how to write the system prompt, pure-declarative no-execute principle, onActivate only for dynamic per-user tools.
• memory — separate sections for ChatMemory and VectorMemory backends, invariant lists.
• rag — composition-first ("can this be done by wrapping Retriever?"), keep createRAG small.
• observability — observers are read-only; mutation belongs in a wrapper runtime.
• sandbox — deny-by-default network/filesystem, security posture non-negotiable.
• eval — scores in [0, 1], every metric optional, replay-first design.
• templates — helpers as pure functions, defaults in JSDoc.

Why this matters for Fase 3 onboarding

Fase 3 adds ~10 new adapters, ~15 new tools, 8 memory backends, 4 UI frameworks. Without CONVENTIONS.md per package, every contributor relitigates the same design questions. These docs are the reference for 'how do I add X to Y?'.

Test plan

• 14 CONVENTIONS.md files created
• Each links to the relevant ADRs and sibling packages
• Each ends with a package-specific review checklist
• Reviewer: spot-check the rules against real patterns in each src/
• Reviewer: confirm the stability-tier notes match what was declared in feat: declare stability tier per package [closes #227] #279
• Reviewer: any pitfall missing from your experience that should land now?

Follow-up

The linked 'contract test suites' (AdapterContractSuite, MemoryContractSuite) referenced in several CONVENTIONS don't exist yet — they're tracked for Phase 2 work.

Refs #232 #211