feat(serve): add workspace MCP management

docs/design/mcp-management-runtime-model.md

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+# MCP Management Runtime Model
+
+MCP configuration is the durable source of truth. Each CLI or Web session
+continues to own an independent MCP runtime so the CLI does not depend on a
+workspace management process.
+
+The Web management page may create an optional management runtime for status
+and management operations. Configuration-changing operations persist first,
+then reconcile every live runtime in the same ACP process. A later session
+loads the persisted configuration normally.
+
+Management status is read from the management runtime's client manager, not
+from the process-wide compatibility status map. The compatibility map remains
+unchanged for existing CLI consumers. Shared-pool reconnects restart the pool
+entry; non-pooled reconnects rediscover the server in each live runtime.
+
+Server provenance remains distinct: user settings, workspace settings,
+project `.mcp.json`, and extensions. Disabling project or workspace servers
+writes the exclusion to workspace-local settings without modifying the shared
+project file.