feat(setup): add Linear channel, fix setup permissions

Enable Linear channel adapter. Fix setup permission rules: use specific
npm install entries per adapter package, replace cp -r with rsync -a to
avoid built-in cp safety prompt, add head to allow list for chained
commands. Update Linear API key URL.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/channels/index.ts

@@ -14,7 +14,7 @@ import './telegram.js';
 // import './github.js';
 
 // linear
-// import './linear.js';
+import './linear.js';
 
 // google chat
 // import './gchat.js';