[appkit plugin] Add @databricks/appkit-agent package

[hubertzub-db]

🥞 Stacked PR

• stack/agent-plugin-p2
• stack/agent-plugin-p3 (diff)
• stack/agent-plugin-p4 (diff)
• stack/agent-plugin-p5 (diff)

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Summary

Extracts the agent plugin from the AppKit monorepo into a standalone, publishable package at integrations/appkit-agent.

This package provides:

1. AgentInterface — a contract for writing custom agent implementations that speak the OpenAI Responses API format (streaming + non-streaming).
2. StandardAgent — a ready-to-use LangGraph-based ReAct agent with function tools and Databricks-hosted tool integration (Genie, Vector Search, MCP servers).

Key decisions:

• @databricks/appkit is a peer dependency — the plugin integrates via AppKit's public Plugin/toPlugin/PluginManifest API
• LangChain packages (@databricks/langchainjs, @langchain/core, @langchain/langgraph, @langchain/mcp-adapters) are optional peer dependencies — only needed when using StandardAgent, not needed for custom agentInstance implementations
• Internal createLogger replaced with a local minimal console logger to avoid coupling to AppKit internals
• Added CI workflow (appkit-agent.yml) and release workflow (release-appkit-agent.yml)

[smurching]

Looks pretty good (mostly just a copy-over of the code from databricks/appkit#166), but should we also include tests here/those seem missing in this PR?

[hubertzub-db]

but should we also include tests here/those seem missing in this PR?

@smurching agree, let's bring over tests in this PR. Initially I wanted to bring them over in latter parts but there's no guarantee we'll get there soon - so it sounds good to colocate them here.
I've added appkit test harness, moved unit tests and added one larger integration-ish test.