feat(appkit): reference agent-app, dev-playground chat UI, docs, and template

[MarioCadenas]

Final layer of the agents feature stack. Everything needed to
exercise, demonstrate, and learn the feature.

Reference application: agent-app

apps/agent-app/ — a standalone app purpose-built around the agents
feature. Ships with:

• server.ts — full example of code-defined agents via fromPlugin:

  const support = createAgent({
    instructions: "…",
    tools: {
      ...fromPlugin(analytics),
      ...fromPlugin(files),
      get_weather,
      "mcp.vector-search": mcpServer("vector-search", "https://…"),
    },
  });
  
  await createApp({
    plugins: [server({ port }), analytics(), files(), agents({ agents: { support } })],
  });

• config/agents/assistant.md — markdown-driven agent alongside the
  code-defined one, showing the asymmetric auto-inherit default.
• Vite + React 19 + TailwindCSS frontend with a chat UI.
• Databricks deployment config (databricks.yml, app.yaml) and
  deploy scripts.

dev-playground chat UI + demo agent

apps/dev-playground/client/src/routes/agent.route.tsx — chat UI with
inline autocomplete (hits the autocomplete markdown agent) and a
full threaded conversation panel (hits the default agent).

apps/dev-playground/server/index.ts — adds a code-defined helper
agent using fromPlugin(analytics) alongside the markdown-driven
autocomplete agent in config/agents/. Exercises the mixed-style
setup (markdown + code) against the same plugin list.

apps/dev-playground/config/agents/*.md — both agents defined with
valid YAML frontmatter.

Docs

docs/docs/plugins/agents.md — progressive five-level guide:

1. Drop a markdown file → it just works.
2. Scope tools via toolkits: / tools: frontmatter.
3. Code-defined agents with fromPlugin().
4. Sub-agents.
5. Standalone runAgent() (no createApp or HTTP).

Plus a configuration reference, runtime API reference, and frontmatter
schema table.

docs/docs/api/appkit/ — regenerated typedoc for the new public
surface (fromPlugin, runAgent, AgentDefinition, AgentsPluginConfig,
ToolkitEntry, ToolkitOptions, all adapter types, and the agents
plugin factory).

Template

template/appkit.plugins.json — adds the agent plugin entry so
npx @databricks/appkit init --features agent scaffolds the plugin
correctly.

Test plan

• Full appkit vitest suite: 1311 tests passing
• Typecheck clean across all 8 workspace projects
• pnpm docs:build clean (no broken links)
• pnpm --filter=@databricks/appkit build:package clean, publint
  clean

Signed-off-by: MarioCadenas MarioCadenas@users.noreply.github.com

PR Stack

1. Shared agent types + LLM adapters — feat(appkit): shared agent types and LLM adapter implementations #301
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 (this PR)

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.