feat(m7.1): TypeScript port — types, Model, FakeModel

[shamspias]

Starts Phase 7. The TS package mirrors the Python and Go verbs and will pass the same shared conformance suite (spec/conformance) by M7.5.

• packages/js (package "reins", ESM + NodeNext), strict TypeScript: strict + noUncheckedIndexedAccess + verbatimModuleSyntax — the tsc equivalent of the Python mypy --strict bar.
• src/types.ts: the core value types (Message, ToolCall, ModelResponse, Usage, FinishReason, Access, Capability, ToolSpec, CapabilityResult, Context, RunResult, RunState, TableInfo, SchemaIndex, TraceEvent, ModelParams). Fields are snake_case to match the contract wire format and mainstream LLM APIs (OpenAI's own JSON uses finish_reason / tool_calls / input_schema), so JSON serialization is identity and every conformance case maps straight on with no renaming. toolSpec() extracts the intent surface — policy metadata (access/confirm/scope) never crosses to the model (§2.5).
• src/errors.ts: ReinsError carrying a → hint (§2.19), instanceof-safe.
• src/model.ts: the Model interface (async — real adapters do network I/O; the loop awaits) and the scriptable, deterministic FakeModel (finalResponse / callResponse / programResponse), recording every call's inputs and rejecting with a hinted error once the script is exhausted — the determinism boundary made concrete.
• tests (vitest): toolSpec strips policy metadata, JSON round-trips, result helpers, and FakeModel ordering / input-snapshotting / exhaustion / builder shapes.
• make js-check (prettier + tsc --noEmit + vitest) as the TS gate + a separate js CI job (Node 20); package-lock.json committed for reproducible installs.

Verified: make js-check green (format, typecheck, 13 tests) and the built dist imports cleanly as ESM from Node (VERSION 0.1.0a1, matching the other packages).