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/tsconfig.json

@@ -0,0 +1,8 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "outDir": "./dist",
+    "rootDir": "./src"
+  },
+  "include": ["src/**/*.ts"]
+}