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/package.json

@@ -0,0 +1,25 @@
+{
+  "name": "@pubscript/ai",
+  "version": "0.1.0",
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "scripts": {
+    "dev": "echo \"@pubscript/ai has no dev server\" (exit 0)",
+    "build": "tsc --noEmit",
+    "test": "vitest run",
+    "lint": "eslint .",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "@pubscript/core": "workspace:*"
+  },
+  "devDependencies": {
+    "@types/node": "^22.10.2",
+    "typescript": "^5.7.2",
+    "vitest": "^2.1.8"
+  }
+}