chore: point every in-tree reference at ai-job-search-dev/ai-job-search (#776)

The repository is moving from the MadsLorentzen personal account to the
ai-job-search-dev organization. GitHub redirects the old slug, but the
upstream-watch guard compares github.repository against the literal slug
and would start running the fork-only triage job on upstream itself, and
the upstream-check tool and its tests pin the slug too. README, CONTRIBUTING,
SECURITY, SETUP, the CI workflow, the issue-template config and the
CHANGELOG links follow.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

tools/check_upstream_updates.py

@@ -35,7 +35,7 @@ FRAMEWORK_FILES = [
     "AGENTS.md",
 ]
 
-UPSTREAM_REPO_SLUG = "MadsLorentzen/ai-job-search"
+UPSTREAM_REPO_SLUG = "ai-job-search-dev/ai-job-search"
 
 def run_git(args: list[str]) -> tuple[int, str, str]:
     res = subprocess.run(