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-slack-v2/SKILL.md

@@ -16,7 +16,7 @@ Check if `src/channels/slack.ts` exists and the import is uncommented in `src/ch
 ### Install the adapter package
 
 ```bash
-npm install @chat-adapter/slack
+pnpm install @chat-adapter/slack
 ```
 
 ### Enable the channel
@@ -30,7 +30,7 @@ import './slack.js';
 ### Build
 
 ```bash
-npm run build
+pnpm run build
 ```
 
 ## Credentials