feat(appkit): shared agent types and LLM adapter implementations

[MarioCadenas]

Foundation layer for the agents feature. Adds the portable type surface
that every downstream layer builds on, plus three LLM adapter
implementations so the agents plugin (later PR) can target whatever the
user has.

Shared agent types

packages/shared/src/agent.ts — no behavior, just the type vocabulary:
AgentAdapter, AgentEvent, AgentInput, AgentRunContext,
AgentToolDefinition, Message, Thread, ThreadStore, ToolAnnotations,
ToolCall, ToolProvider, ResponseStreamEvent. Exported from the
shared barrel.

Adapters

• packages/appkit/src/agents/databricks.ts — DatabricksAdapter:
  streams OpenAI-compatible completions against a Databricks Model
  Serving endpoint (raw fetch + SSE, no vendor SDKs).
• packages/appkit/src/agents/vercel-ai.ts — VercelAIAdapter:
  wraps any Vercel AI SDK streamText call. Maps Vercel SDK events to
  AppKit AgentEvents and tool calls.
• packages/appkit/src/agents/langchain.ts — LangChainAdapter:
  wraps any LangChain Runnable (AgentExecutor, compiled LangGraph,
  etc.). Subscribes to streamEvents(v2) and maps to AgentEvents.

Each adapter is self-contained and independently testable.

Package plumbing

• Subpath exports @databricks/appkit/agents/{databricks,vercel-ai,langchain}
  so consumers pick only the adapter they want.
• @langchain/core and ai declared as optional peer dependencies.
• @ai-sdk/openai, @langchain/core, ai added as devDeps for tests.
• tsdown.config.ts emits the three adapter entry points alongside the
  main bundle.

Test plan

• 24 adapter tests (Databricks: 16, Vercel AI: 4, LangChain: 4) passing
• Full appkit vitest suite: 1154 tests passing

PR Stack

1. Shared agent types + LLM adapters (this PR)
2. Tool primitives + ToolProvider surfaces — feat(appkit): tool primitives and ToolProvider surfaces on core plugins #302
3. Plugin infrastructure (attachContext + PluginContext) — feat(appkit): plugin infrastructure — attachContext + PluginContext mediator #303
4. agents() plugin + createAgent(def) + markdown-driven agents — feat(appkit): agents() plugin, createAgent(def), and markdown-driven agents #304
5. fromPlugin() DX + runAgent plugins arg + toolkit-resolver — feat(appkit): fromPlugin() DX, runAgent plugins arg, shared toolkit-resolver #305
6. Reference app + dev-playground + docs — feat(appkit): reference agent-app, dev-playground chat UI, docs, and template #306

This 6-PR stack is a redesign of the earlier 13-PR stack (#282-#286, #293-#300), reorganized to eliminate mid-stack API churn. Reviewers never see code in this stack that later PRs delete. The old 13 PRs have been closed with references back here.