fix(rename): surface duplicate-slug 409 instead of 500 (v1.21.1)

mesh.slug actually carries a UNIQUE constraint (mesh_slug_unique)
even though the schema comment claimed otherwise. Trying to rename
to a slug another mesh already owns blew up as a generic 500.
Now: caught at the route, surfaced as 409 with body
{"error":"slug \"<x>\" is already taken"}; CLI maps it to
EXIT.ALREADY_EXISTS and prints the message.

Schema comment corrected to match DB reality.

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

apps/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "claudemesh-cli",
-  "version": "1.21.0",
+  "version": "1.21.1",
   "description": "Peer mesh for Claude Code sessions — CLI + MCP server.",
   "keywords": [
     "claude-code",