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. The fromServingEndpoint / fromModelServing
  factories route through the shared connectors/serving/stream
  helper, which delegates to the SDK's apiClient.request({ raw: true }).
  That gives the adapter centralised URL encoding + authentication
  with the rest of the serving surface — no bespoke fetch() +
  authenticate() plumbing. The raw new DatabricksAdapter({ endpointUrl, authenticate }) constructor is preserved as an escape hatch for
  tests and for pointing the adapter at non-workspace endpoints.

Each adapter is self-contained and independently testable.

Test plan

• 17 adapter tests (Databricks) covering raw-fetch escape hatch,
  serving-connector routing, URL encoding of endpoint names with
  special characters, streaming + tool-call dispatch.
• Full appkit vitest suite: 1552 tests passing at stack tip.

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

Demo

agent-demo.mp4