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.

pnpm-lock.yaml

@@ -85,6 +85,22 @@ importers:
         specifier: ^2.1.8
         version: 2.1.9(@types/node@22.20.5)(jsdom@25.0.1)
 
+  packages/ai:
+    dependencies:
+      '@pubscript/core':
+        specifier: workspace:*
+        version: link:../core
+    devDependencies:
+      '@types/node':
+        specifier: ^22.10.2
+        version: 22.20.5
+      typescript:
+        specifier: ^5.7.2
+        version: 5.9.3
+      vitest:
+        specifier: ^2.1.8
+        version: 2.1.9(@types/node@22.20.5)(jsdom@25.0.1)
+
   packages/core:
     dependencies:
       dexie: