feat(appkit): plugin infrastructure — attachContext + PluginContext mediator

[MarioCadenas]

Third layer: the substrate every downstream PR relies on. No user-
facing API changes here; the surface for this PR is the mediator
pattern, lifecycle semantics, and factory stamping.

Split Plugin construction from context binding

Plugin constructors become pure — no CacheManager.getInstanceSync(),
no TelemetryManager.getProvider(), no PluginContext wiring inside
constructor(). That work moves to a new lifecycle method:

interface BasePlugin {
  attachContext?(deps: {
    context?: unknown;
    telemetryConfig?: TelemetryOptions;
  }): void;
}

createApp calls attachContext() on every plugin after all
constructors have run, before setup(). This lets factories return
PluginData tuples at module scope without pulling core services into
the import graph — a prerequisite for later PRs that construct agent
definitions before createApp.

PluginContext mediator

packages/appkit/src/core/plugin-context.ts — new class that mediates
all inter-plugin communication:

• Route buffering: addRoute() / addMiddleware() buffer until
  the server plugin calls registerAsRouteTarget(), then flush via
  addExtension(). Eliminates plugin-ordering fragility.
• ToolProvider registry: registerToolProvider(name, plugin) +
  live getToolProviders(). Typed discovery of tool-exposing plugins.
• User-scoped tool execution: executeTool(req, pluginName, localName, args, signal?) resolves the provider, wraps in
  asUser(req) for OBO, opens a telemetry span, applies a 30s
  timeout, dispatches, returns.
• Lifecycle hooks: onLifecycle('setup:complete' | 'server:ready' | 'shutdown', cb) + emitLifecycle(event). Callback errors don't
  block siblings.

toPlugin stamps pluginName

packages/appkit/src/plugin/to-plugin.ts — the factory now attaches a
read-only pluginName property to the returned function. Later PRs'
fromPlugin(factory) reads it to identify which plugin a factory
refers to without needing to construct an instance. NamedPluginFactory
type exported for consumers who want to type-constrain factories.

Server plugin defers start to setup:complete

ServerPlugin.setup() no longer calls extendRoutes() synchronously.
It subscribes to the setup:complete lifecycle event via
PluginContext and starts the HTTP server there. This ensures that
any deferred-phase plugin (agents plugin in a later PR) has had a
chance to register routes via PluginContext.addRoute() before the
server binds. Removes the plugins field from ServerConfig (routes
are now discovered via the context, not a config snapshot).

Test plan

• 25 new PluginContext tests (route buffering, tool provider registry,
  executeTool paths, lifecycle hooks, plugin metadata)
• Updated AppKit lifecycle tests to inject context instead of
  plugins
• Full appkit vitest suite: 1237 tests passing
• Typecheck clean across all 8 workspace projects

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) (this PR)
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): tools(plugins) 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

[pkosiec]

LGTM but please verify the agentic review findings before merge 👍