feat(plugins): reach a running agent's composer by reference

useComposers(selector) in any plugin component and composers(selector) on every command context return the composer of a running agent, named by id or by focus, whether or not its tab is open.

A running agent now has one store-backed composer model apart from any mounted tab. The mounted composer plugs its cursor-aware insert in while mounted; otherwise insertText and setPrompt write the agent's saved prompt. model and thinking resolve through the provider catalog with the picker's own function, so a read equals what the picker shows.

The host reads one ComposerModel from context and no longer knows whether it is in an agent, a draft or New workspace. The daemon rejects an unlisted model or thinking option for a running agent, replacing the app-side guard.

packages/plugin/src/client/contracts.ts

@@ -4,7 +4,11 @@ import type { PaseoApi } from "@getpaseo/client";
 import type { AgentTimelineItem, JsonValue } from "@getpaseo/protocol/agent-types";
 import type { ZodType, input as ZodInput, output as ZodOutput } from "zod";
 import type { PluginFile } from "./file.js";
-import type { PluginComposerItemContribution } from "./composer.js";
+import type {
+  PluginComposer,
+  PluginComposerItemContribution,
+  PluginComposerSelector,
+} from "./composer.js";
 import type { PluginNewWorkspaceItemContribution } from "./new-workspace.js";
 import type {
   PluginRpcContract,
@@ -318,6 +322,8 @@ export interface PluginCommandCapabilities {
     input: PluginWireInput<ZodInput<InputSchema>>,
   ): Promise<ZodOutput<OutputSchema>>;
   openScreen(input: PluginOpenScreenInput): void;
+  /** The composers `selector` names, whether or not their tabs are open; see `useComposers`. */
+  composers(selector: PluginComposerSelector): readonly PluginComposer[];
   /** @deprecated Use `openScreen`. */
   openSurface(id: string): void;
   openSettings(id: string): void;