feat(mcp): add mcp_read meta-tool for resource reads

Implements plans/mcp-resources-prompts-design.md section 4.3 (T4):
mcp_read_<server> declaration and handler wired to render.rs, RFC 6570
Level-1-only URI template expansion, defensive ResourceContents parsing,
per-item text paging with pattern filtering, blob spill metadata, an
overall 204800-byte multi-content ceiling, dispatch wiring on both
eval chains, and a render_text paging-stall guard.

src/function/supervisor.rs

@@ -1526,9 +1526,10 @@ mod tests {
         sync_agent_functions_to_ctx(&mut ctx).unwrap();
 
         let functions = &ctx.tool_scope.functions;
-        assert_eq!(functions.declarations().len(), 2);
+        assert_eq!(functions.declarations().len(), 3);
         assert!(functions.contains("mcp_search_fixture"));
         assert!(functions.contains("mcp_describe_fixture"));
+        assert!(functions.contains("mcp_read_fixture"));
         assert!(!functions.contains("mcp_invoke_fixture"));
     }