fix(agents): make injected MCP launch requirements explicit

Optional injected servers must survive persistence and reload without turning missing transport support into launch refusal. Required external and plugin servers keep their existing refusal. Advertise the optional MCP contract so older hosts retain established strict launch behavior.

packages/protocol/src/agent-types.ts

@@ -515,6 +515,8 @@ export interface ToolPolicy {
 }
 
 export interface AgentSessionConfig {
+  /** Per-server launch requirement; unspecified servers are required. */
+  mcpServerRequirements?: Record<string, "optional" | "required">;
   provider: AgentProvider;
   cwd: string;
   /**

packages/protocol/src/messages.ts

@@ -494,6 +494,7 @@ const AgentSessionConfigSchema = z.object({
     enforcement: z.enum(["required", "if-supported"]).optional(),
   }).optional(),
   systemPrompt: z.string().optional(),
+  mcpServerRequirements: z.record(z.string(), z.enum(["optional", "required"])).optional(),
   mcpServers: z.record(z.string(), McpServerConfigSchema).optional(),
 });
 
@@ -3673,6 +3674,7 @@ export const ServerInfoStatusPayloadSchema = z
         internalWorkspaces: z.boolean().optional(),
         backgroundWorkspaces: z.boolean().optional(),
         providersSnapshot: z.boolean().optional(),
+        optionalMcpSupport: z.boolean().optional(),
         connectedHosts: z.boolean().optional(),
         hostMultiplexing: z.boolean().optional(),
         usageSources: z.boolean().optional(),