feat: remove old installer, update docs to npx claude-mem

Removes installer/ directory (16 files) — fully replaced by src/npx-cli/.
Updates install.sh and installer.js to redirect to npx claude-mem.
Adds npx claude-mem as primary install method in docs and README.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/npx-cli/commands/install.ts

@@ -30,7 +30,7 @@ import {
 import { detectInstalledIDEs } from './ide-detection.js';
 
 // ---------------------------------------------------------------------------
-// Registration helpers (mirror installer/src/steps/install.ts)
+// Registration helpers
 // ---------------------------------------------------------------------------
 
 function registerMarketplace(): void {