feat(appkit): fromPlugin() DX, runAgent plugins arg, shared toolkit-resolver

[MarioCadenas]

DX centerpiece. Introduces the symbol-marker pattern that collapses
plugin tool references in code-defined agents from a three-touch dance
to a single line, and extracts the shared resolver that the agents
plugin, auto-inherit, and standalone runAgent all now go through.

fromPlugin(factory, opts?) — the marker

packages/appkit/src/plugins/agents/from-plugin.ts. Returns a spread-
friendly { [Symbol()]: FromPluginMarker } record. The symbol key is
freshly generated per call, so multiple spreads of the same plugin
coexist safely. The marker's brand is a globally-interned
Symbol.for("@databricks/appkit.fromPluginMarker") — stable across
module boundaries.

resolveToolkitFromProvider(pluginName, provider, opts?)

packages/appkit/src/plugins/agents/toolkit-resolver.ts. Single source
of truth for "turn a ToolProvider into a keyed record of ToolkitEntry
markers". Prefers provider.toolkit(opts) when available (core plugins
implement it), falls back to walking getAgentTools() and synthesizing
namespaced keys (${pluginName}.${localName}) for third-party
providers, honoring only / except / rename / prefix the same
way.

Used by three call sites, previously all copy-pasted:

1. AgentsPlugin.buildToolIndex — fromPlugin marker resolution pass
2. AgentsPlugin.applyAutoInherit — markdown auto-inherit path
3. runAgent — standalone-mode plugin tool dispatch

AgentsPlugin.buildToolIndex — symbol-key resolution pass

Before the existing string-key iteration, buildToolIndex now walks
Object.getOwnPropertySymbols(def.tools). For each FromPluginMarker,
it looks up the plugin by name in PluginContext.getToolProviders(),
calls resolveToolkitFromProvider, and merges the resulting entries
into the per-agent index. Missing plugins throw at setup time with a
clear Available: ... listing — wiring errors surface on boot, not
mid-request.

hasExplicitTools now counts symbol keys too, so a
tools: { ...fromPlugin(x) } record correctly disables auto-inherit
on code-defined agents.

Type plumbing

• AgentTools type: { [key: string]: AgentTool } & { [key: symbol]: FromPluginMarker }. Preserves string-key autocomplete while
  accepting marker spreads under strict TS.
• AgentDefinition.tools switched to AgentTools.

runAgent gains plugins?: PluginData[]

packages/appkit/src/core/run-agent.ts. When an agent def contains
fromPlugin markers, the caller passes plugins via
RunAgentInput.plugins. A local provider cache constructs each plugin
and dispatches tool calls via provider.executeAgentTool(). Runs as
service principal (no OBO — there's no HTTP request). If a def
contains markers but plugins is absent, throws with guidance.

Exports

fromPlugin, FromPluginMarker, isFromPluginMarker, AgentTools
added to the main barrel.

Test plan

• 14 new tests: marker shape, symbol uniqueness, type guard,
  factory-without-pluginName error, fromPlugin marker resolution in
  AgentsPlugin, fallback to getAgentTools for providers without
  .toolkit(), symbol-only tools disables auto-inherit, runAgent
  standalone marker resolution via plugins arg, guidance error when
  missing.
• Full appkit vitest suite: 1311 tests passing.
• Typecheck 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 (this PR)
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.