feat: claudemesh slug <old> <new> — change a mesh's slug (v1.20.0)

Slugs are not globally unique (mesh.id is canonical) so the route
only validates the regex and updates the row. CLI refuses a local
collision (two joined meshes sharing a slug would make the picker
ambiguous) and rewrites ~/.claudemesh/config.json on success.
Other peers pick up the new slug on next claudemesh sync.

Server: PATCH /api/cli/meshes/:slug body now accepts { name?, slug? }
— same route, just optional both fields.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "claudemesh-cli",
-  "version": "1.19.3",
+  "version": "1.20.0",
   "description": "Peer mesh for Claude Code sessions — CLI + MCP server.",
   "keywords": [
     "claude-code",