fix: strip mcp_ prefix from tool names before dispatch

The model emits tool names like mcp_background_output but the runtime
registry has them as background_output. While transformToolName already
handles the prefix for display purposes, the tool dispatch path in
tool-execute-before was not stripping it, causing 'unavailable tool' errors.

This adds mcp_ prefix stripping at the earliest point in the tool
execution pipeline, fixing background_output, background_cancel, and
all nocturne-memory_* tools.

Closes #2697

src/plugin/tool-execute-before.ts

@@ -52,6 +52,19 @@ export function createToolExecuteBeforeHandler(args: {
   }
 
   return async (input, output): Promise<void> => {
+    // Strip mcp_ prefix from tool names — the model may emit mcp_background_output
+    // but the runtime registry has it as background_output (fixes #2697)
+    if (/^mcp_/i.test(input.tool)) {
+      const stripped = input.tool.replace(/^mcp_/i, "")
+      log("[tool-execute-before] Stripped mcp_ prefix from tool name", {
+        original: input.tool,
+        resolved: stripped,
+        sessionID: input.sessionID,
+        callID: input.callID,
+      })
+      input.tool = stripped
+    }
+
     if (input.tool.toLowerCase() === "bash" && typeof output.args.command === "string") {
       if (output.args.command.includes("\x00")) {
         replaceToolArgs(output, { command: output.args.command.replace(/\x00/g, "") })