refactor: normalize apply statuses, remove dead code, fix signal handler

- lib/apply/index.mjs: add STATUS_MAP to normalize platform-specific statuses
  to generic ones (no_button/no_submit/no_modal → skipped_no_apply).
  Documented all generic statuses for AI/developer reference.
- job_applier.mjs: handleResult now handles skipped_no_apply, default case
  logs + saves instead of silently dropping
- lib/linkedin.mjs: remove dead applyLinkedIn() and detectAts(), clean imports
  (~110 lines removed). Search-only module now.
- lib/wellfound.mjs: remove dead applyWellfound(), clean imports.
  Search-only module now.
- lib/lock.mjs: fix async signal handler — shutdown handlers now actually
  complete before process.exit()
- test_linkedin_login.mjs: add try/catch/finally with proper browser cleanup
- README: update status table with all current statuses

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

job_applier.mjs

@@ -199,11 +199,11 @@ async function handleResult(job, result, results, settings) {
       break;
     }
 
+    case 'skipped_no_apply':
     case 'skipped_easy_apply_unsupported':
     case 'skipped_honeypot':
     case 'stuck':
     case 'incomplete':
-    case 'no_modal':
       console.log(`    ⏭️  Skipped — ${status}`);
       updateJobStatus(job.id, status, { title, company });
       appendLog({ ...job, title, company, status });
@@ -211,7 +211,9 @@ async function handleResult(job, result, results, settings) {
       break;
 
     default:
-      console.log(`    ⚠️  Unknown status: ${status}`);
+      console.warn(`    ⚠️  Unhandled status: ${status}`);
+      updateJobStatus(job.id, status, { title, company });
+      appendLog({ ...job, title, company, status });
   }
 }