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/errors.ts

@@ -0,0 +1,49 @@
+/**
+ * Error types for the AI provider layer.
+ *
+ * Errors are classified so callers can react appropriately: auth failures are
+ * terminal (the key is wrong), rate-limit/overloaded errors are retryable, and
+ * budget violations are a client-side guard.
+ *
+ * See CHUNK#8.md §3.
+ */
+
+/** Base class for all provider-layer errors. */
+export class ProviderError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "ProviderError";
+  }
+}
+
+/** The provider rejected the request due to missing/invalid credentials. */
+export class ProviderAuthError extends ProviderError {
+  constructor(message = "Provider authentication failed (invalid API key).") {
+    super(message);
+    this.name = "ProviderAuthError";
+  }
+}
+
+/** The provider rate-limited or overloaded the request; safe to retry. */
+export class ProviderRateLimitError extends ProviderError {
+  constructor(message = "Provider rate limit exceeded; retrying.") {
+    super(message);
+    this.name = "ProviderRateLimitError";
+  }
+}
+
+/** The request exceeded the configured token budget. */
+export class BudgetExceededError extends ProviderError {
+  constructor(message: string) {
+    super(message);
+    this.name = "BudgetExceededError";
+  }
+}
+
+/** No provider adapter is registered for the given kind. */
+export class UnknownProviderError extends ProviderError {
+  constructor(kind: string) {
+    super(`Unknown provider kind: ${kind}`);
+    this.name = "UnknownProviderError";
+  }
+}