feat(server): add background agents

An agent can be created background in any workspace. It stays a durable,
addressable agent, but default discovery, workspace status and attention
leave it out. Effective background is agent.background ||
workspace.background.

- Protocol: `create_agent_request.background` now marks the agent itself
  and stays the workspace intent for a workspace the request creates.
  Snapshots carry `background`. The daemon advertises
  `features.backgroundAgents`. Clients advertise the `background_agents`
  capability.
- Daemon: the flag persists in the agent record, so restart and resume
  keep it. AgentDirectory owns visibility. Inclusive reads and
  subscriptions from sources without `background_agents` see only agents
  in background workspaces, because older apps would otherwise open a tab
  for each one.
- Inheritance: omitted on agent-scoped creation, children and new
  workspaces take the caller agent's background.
- MCP: create_agent accepts `background`.
- SDK: placing a background agent in an existing workspace, or under a
  caller, fails fast with "Update the host to use background agents." on
  older hosts.
- CLI: `ls -a` and `ls --background` add a BACKGROUND column.

packages/protocol/src/client-capabilities.ts

@@ -51,6 +51,10 @@ export const CLIENT_CAPS = {
   pluginBinaryPayloads: "plugin_binary_payloads",
   // COMPAT(workspaceSetupBlocked): added in v0.8.0, remove after 2027-03-07 once client floor >= v0.8.0.
   workspaceSetupBlocked: "workspace_setup_blocked",
+  // COMPAT(backgroundAgents): added in v0.15.0, remove after 2027-05-03 once client floor >= v0.15.0.
+  // Sources without it never receive agent-level background agents in inclusive lists or
+  // update subscriptions, because older apps auto-open a tab for every replicated root agent.
+  backgroundAgents: "background_agents",
   browserHost: "browser_host",
 } as const;
 

packages/protocol/src/messages.ts

@@ -891,6 +891,9 @@ export const AgentSnapshotPayloadSchema = z.object({
   providerUnavailable: z.boolean().optional(),
   // Private helper marker, retained for exact-ID snapshots and older wire messages.
   internal: z.boolean().optional(),
+  // Background agent: hidden from default discovery, tabs, tracks and notifications.
+  // Fixed at creation. Effective background also includes the workspace's flag.
+  background: z.boolean().optional(),
 });
 
 export type AgentSnapshotPayload = z.infer<typeof AgentSnapshotPayloadSchema>;
@@ -1802,7 +1805,8 @@ export const CreateAgentRequestMessageSchema = z.object({
   // in the config schema, which is reused for updates. Retained for wire parsing;
   // public internal creation is no longer supported.
   internal: z.boolean().optional(),
-  // Workspace creation intent, rejected when selecting an existing workspace.
+  // The created agent is a background agent. A workspace this request creates inherits it.
+  // Omitted on agent-scoped creation, the caller agent's own value applies.
   background: z.boolean().optional(),
   labels: z.record(z.string(), z.string()).default({}),
   requestId: z.string(),
@@ -3694,6 +3698,8 @@ export const ServerInfoStatusPayloadSchema = z
         // COMPAT(internalWorkspaces): added in v0.9.0; remove gate after 2027-03-17.
         internalWorkspaces: z.boolean().optional(),
         backgroundWorkspaces: z.boolean().optional(),
+        // COMPAT(backgroundAgents): added in v0.15.0; remove gate after 2027-05-03.
+        backgroundAgents: z.boolean().optional(),
         providersSnapshot: z.boolean().optional(),
         connectedHosts: z.boolean().optional(),
         hostMultiplexing: z.boolean().optional(),
@@ -7802,6 +7808,7 @@ export const WSHelloMessageSchema = z.object({
       [CLIENT_CAPS.providerSnapshotReferences]: z.boolean().optional(),
       [CLIENT_CAPS.timelineReplacementInvalidation]: z.boolean().optional(),
       [CLIENT_CAPS.timelineNotifications]: z.boolean().optional(),
+      [CLIENT_CAPS.backgroundAgents]: z.boolean().optional(),
       [CLIENT_CAPS.browserHost]: BrowserAutomationHostCapabilitySchema.optional(),
     })
     .passthrough()