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>

job_applier.mjs

@@ -6,8 +6,7 @@ loadEnv(); // load .env before anything else
  * Reads jobs queue and applies using the appropriate handler per apply_type
  * Run via cron or manually: node job_applier.mjs [--preview]
  */
-import { existsSync } from 'fs';
-import { writeFileSync } from 'fs';
+import { existsSync, writeFileSync } from 'fs';
 import { dirname, resolve } from 'path';
 import { fileURLToPath } from 'url';