feat(plugins): agents and workspaces carry plugin metadata

Plugins had nowhere to keep their own data on an agent or workspace, so
Phase 5 and 6 surfaces would each have invented a side store. Both now
hold an opaque Record<string, JsonValue> that Paseo stores, snapshots and
hands to hooks without reading a key.

Set at creation or changed with agent.metadata.update.request /
workspace.metadata.update.request (top-level keys replace, null removes).
Persisted on the agent and workspace records, so it survives a restart.
Reaches agent.create.before (which may add to it), agent.open.before
(read-only), agent.create.after and workspace.create.before/after.

Gated once on server_info.features.entityMetadata; the client throws
"Update the host" instead of sending to a host without it.

packages/plugin/src/server/lifecycle.ts

@@ -4,6 +4,7 @@ import type {
   AgentPermissionResponse,
   AgentTimelineItem,
   AgentSessionConfig,
+  JsonValue,
 } from "@getpaseo/protocol/agent-types";
 import type { PaseoApi } from "@getpaseo/client";
 import type { WorkspaceCreateRequest } from "@getpaseo/protocol/messages";
@@ -19,6 +20,8 @@ export interface PluginHookWorkspace {
   cwd: string;
   name: string | null;
   archivedAt: string | null;
+  /** Data plugins stored on the workspace. Paseo never reads a key. */
+  metadata: Record<string, JsonValue>;
 }
 
 export interface PluginHookWorktree {
@@ -68,6 +71,8 @@ export interface PluginHookAgent {
   provider: string;
   cwd: string;
   title: string | null;
+  /** Data plugins stored on the agent. Paseo never reads a key. */
+  metadata: Record<string, JsonValue>;
 }
 
 export interface PluginSessionOpenRequest {
@@ -79,6 +84,7 @@ export interface PluginSessionOpenRequest {
   purpose: "interactive" | "history";
   readonly title: string | null;
   readonly labels: Readonly<Record<string, string>>;
+  readonly metadata: Readonly<Record<string, JsonValue>>;
   tools: AgentToolDefinition[];
   env: Record<string, string>;
   mcpServers: NonNullable<AgentSessionConfig["mcpServers"]>;
@@ -116,7 +122,12 @@ export interface PluginHookEvents {
 
 export interface PluginHookRequests {
   "worktree.create.before": PluginWorktreeCreateRequest;
-  "agent.create.before": { config: AgentSessionConfig; env?: Record<string, string> };
+  "agent.create.before": {
+    config: AgentSessionConfig;
+    env?: Record<string, string>;
+    /** Stored on the new agent. Always present in the payload; return it changed to change it. */
+    metadata?: Record<string, JsonValue>;
+  };
   "agent.open.before": PluginSessionOpenRequest;
   "workspace.create.before": Omit<WorkspaceCreateRequest, "type" | "requestId">;
 }