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
YeonGyu-Kim