Add scratchpad_delete MCP tool

Mirrors the existing scratchpad_* tools end-to-end: catalog schema,
dispatch, ToolHost.ScratchpadDelete, and a host method that delegates to
scratchpad.Store.Delete and fires scratchpadsChanged() on success so the
sidebar refreshes. Missing-pad errors surface rather than being masked.

Resolves the [MCP SCRATCHPAD DELETE] TODO item.

CHANGELOG.md

@@ -6,6 +6,10 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Added
+- MCP clients can now call `scratchpad_delete` with a scratchpad name
+  to remove a shared project scratchpad.
+
 ### Fixed
 - Sidebar timer indicators now repaint as their visible countdown
   value changes, so labels progress from minutes to seconds without