feat(mcp): gate meta-function emission on advertised server capabilities

Per-server McpServerFeatures (tools fail-open, resources/prompts
fail-closed) now drive which meta-functions are declared, with
gated_meta_function_prefixes as the single gating seam; read/prompt
declarations land together with their handlers. The server-enablement
sentinel keys on the always-emitted search name so resources-only
servers survive role filtering.

Implements plans/mcp-resources-prompts-design.md §4.4/D7 (T5).

src/config/app_state.rs

@@ -70,7 +70,7 @@ impl AppState {
 
         let mut functions = Functions::init(config.visible_tools.as_ref().unwrap_or(&Vec::new()))?;
         if !mcp_registry.is_empty() && config.mcp_server_support {
-            functions.append_mcp_meta_functions(mcp_registry.list_started_servers());
+            functions.append_mcp_meta_functions(mcp_registry.server_features());
         }
 
         let mcp_registry = if mcp_registry.is_empty() {