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/mod.rs

@@ -22,6 +22,8 @@ pub(crate) mod todo;
 mod tool_scope;
 mod update;
 
+#[cfg(test)]
+pub(crate) use self::agent::AgentConfig;
 pub use self::agent::{
     Agent, AgentVariable, AgentVariables, complete_agent_variables, list_agents,
     list_agents_with_descriptions,