docs: convert all skill instructions from npm to pnpm

Batch update 62 files across .claude/skills/ — SKILL.md, REMOVE.md,
and script files. Conversions: npm run -> pnpm run, npm install ->
pnpm install, npx -> pnpm exec/dlx, npm uninstall -> pnpm uninstall,
package-lock.json -> pnpm-lock.yaml, shebangs updated.

.claude/skills/add-matrix-v2/SKILL.md

@@ -14,7 +14,7 @@ Check if `src/channels/matrix.ts` exists and the import is uncommented in `src/c
 ## Install
 
 ```bash
-npm install @beeper/chat-adapter-matrix
+pnpm install @beeper/chat-adapter-matrix
 ```
 
 Uncomment the Matrix import in `src/channels/index.ts`:
@@ -24,7 +24,7 @@ import './matrix.js';
 ```
 
 ```bash
-npm run build
+pnpm run build
 ```
 
 ## Credentials