refactor: extract magic numbers to constants, fix audit issues

- Centralize all magic numbers/strings in lib/constants.mjs
- Fix double-replaced import names in filter.mjs
- Consolidate duplicate fs imports in job_applier/job_searcher
- Remove empty JSDoc block in job_searcher
- Update keywords.mjs model from claude-3-haiku to claude-haiku-4-5
- Extract Anthropic API URLs to constants
- Convert :has-text() selectors to page.locator() API
- Fix SIGTERM handler conflict — move partial-run notification into lock.onShutdown
- Remove unused exports (LOCAL_USER_AGENT, DEFAULT_REVIEW_WINDOW_MINUTES)
- Fix variable shadowing (b -> v) in job_filter reduce callback
- Replace SKILL.md PM2 references with system cron

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

lib/apply/wellfound.mjs

@@ -16,8 +16,8 @@ export async function apply(page, job, formFiller) {
     company: document.querySelector('[class*="company"] h2, [class*="startup"] h2, h2')?.textContent?.trim(),
   }));
 
-  const applyBtn = await page.$('a:has-text("Apply"), button:has-text("Apply Now"), a:has-text("Apply Now")');
-  if (!applyBtn) return { status: 'no_button', meta };
+  const applyBtn = page.locator('a:has-text("Apply"), button:has-text("Apply Now"), a:has-text("Apply Now")').first();
+  if (await applyBtn.count() === 0) return { status: 'no_button', meta };
 
   await applyBtn.click();
   await page.waitForTimeout(FORM_FILL_WAIT);