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/init.mjs

@@ -9,7 +9,7 @@
  */
 
 import { readFileSync, existsSync } from 'fs';
-import { resolve } from 'path';
+import { resolve, basename } from 'path';
 import { readProjects } from './shared.mjs';
 
 const cwd = process.env.PWD || process.cwd();
@@ -137,7 +137,7 @@ if (readme && !output.description) {
 
 // ── Name fallback: directory name ─────────────────────────────────────────────
 if (!output.name) {
-  output.name = cwd.split('/').pop().toLowerCase().replace(/\s+/g, '-');
+  output.name = basename(cwd).toLowerCase().replace(/\s+/g, '-');
 }
 
 console.log(JSON.stringify(output, null, 2));