fix(ck): address Greptile + CodeRabbit review bugs

- Fix read-after-write in session-start.mjs: read prevSession BEFORE
  overwriting current-session.json so unsaved-session detection fires
- Fix shell injection in resume.mjs: replace execSync shell string with
  fs.existsSync for directory existence check
- Fix shell injection in shared.mjs gitSummary: replace nested \$(git ...)
  subshell with a separate runGit() call to get rev count
- Fix displayName never shown: render functions now use ctx.displayName
  ?? ctx.name so user-supplied names show instead of the slug
- Fix renderListTable: uses context.displayName ?? entry.name
- Fix init.mjs: use path.basename() instead of cwd.split('/').pop()
- Fix save.mjs confirmation: show original name, not contextDir slug

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

skills/ck/commands/save.mjs

@@ -85,7 +85,7 @@ if (isInit) {
   };
   writeProjects(projects);
 
-  console.log(`✓ Project '${contextDir}' registered.`);
+  console.log(`✓ Project '${name}' registered.`);
   console.log(`  Use /ck:save to save session state and /ck:resume to reload it next time.`);
   process.exit(0);
 }