feat(ai): add packages/ai provider layer (Chunk #8)

Self-contained SSE-based provider layer for OpenAI-compatible endpoints
(Ollama/llama.cpp/LM Studio/vLLM), Anthropic, Gemini, and OpenRouter.

- budget.ts: re-export core token-budget engine (countTokens,
  allocateBudget, DEFAULT_BUDGET)
- client.ts: streamCompletion/collectStream/complete with budget
  enforcement via enforceBudget + measureBudget
- discover.ts: auto-detect local providers by probing common /v1/models
- adapters.ts: per-provider param mapping + streaming adapters + factory
- thinking.ts: thinking/reasoning-param normalization per provider
- http.ts: SSE parsing + 429 retry
- errors.ts: BudgetExceededError

35 unit/integration tests pass; typecheck and lint clean.

packages/ai/src/index.ts

@@ -0,0 +1,72 @@
+/**
+ * `@pubscript/ai` — the AI provider layer for PubScript.
+ *
+ * A self-contained, SSE-based provider abstraction that talks to any
+ * OpenAI-compatible endpoint with streaming, thinking/reasoning params, and
+ * the token-budget engine from `packages/core`. See CHUNK#8.md.
+ *
+ * Public API:
+ * - `streamCompletion`, `collectStream`, `complete` — the completion client.
+ * - `measureBudget`, `enforceBudget` — budget helpers.
+ * - `discoverLocalProviders` — local provider discovery.
+ * - `getProvider` — resolve a provider config into an adapter.
+ * - `normalizeThinking` — thinking-param mapping.
+ * - Re-exports of the token budget engine from `packages/core`.
+ * - All types and error classes.
+ */
+
+// Completion client
+export {
+  streamCompletion,
+  collectStream,
+  complete,
+  measureBudget,
+  enforceBudget,
+} from "./client.js";
+export type { MeasuredBudget, StreamResult } from "./client.js";
+
+// Provider adapters + factory
+export { getProvider } from "./providers/adapters.js";
+
+// Thinking-param mapping
+export { normalizeThinking, reasoningBudgetTokens } from "./thinking.js";
+
+// Local provider discovery
+export {
+  discoverLocalProviders,
+  LOCAL_ENDPOINTS,
+} from "./discover.js";
+export type { DetectedModel, DetectedProvider } from "./discover.js";
+
+// Token budget engine (re-exported from core)
+export {
+  countTokens,
+  allocateBudget,
+  DEFAULT_BUDGET,
+} from "./budget.js";
+export type { Budget } from "./budget.js";
+
+// Types
+export type {
+  ProviderKind,
+  ThinkingConfig,
+  ThinkingEffort,
+  ProviderConfig,
+  ResolvedProvider,
+  Message,
+  CallOptions,
+  CompletionUsage,
+  StreamEventType,
+  StreamChunk,
+  CompleteResult,
+  ProviderAdapter,
+} from "./providers/types.js";
+
+// Errors
+export {
+  ProviderError,
+  ProviderAuthError,
+  ProviderRateLimitError,
+  BudgetExceededError,
+  UnknownProviderError,
+} from "./errors.js";