build: replace ad-hoc run with a full make workflow + helper scripts

The old Makefile was a single 'run' target (hugo server + pagefind --serve).
Replace it with documented targets:

  build         hugo --gc -> public/
  index         build + pagefind --site public -> public/pagefind/
  serve / run   index + pagefind's own server on 127.0.0.1:1414 (local)
  serve-lan     static-serve public/ on 0.0.0.0:8090 for other machines
  link-check    offline internal-link scan (lychee --scheme file) -> JSON
  link-fix      comment out confirmed-dead internal links, then rebuild
  external-check network scan of external links (report-only)
  clean / all

Add scripts/:
  serve_lan.py            stdlib static server rooted at public/ (prints LAN IP)
  fix_dead_links.py       idempotent internal dead-link comment-out
  check_external_links.py external link report (drives the daily cron job)

.gitignore: add linkcheck/, resources/_gen/ (regenerable Hugo asset cache),
__pycache__/, and .hermes/.

scripts/serve_lan.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+"""Serve the built site (public/) on all interfaces so other machines on the
+same LAN can reach it. Usage: python3 serve_lan.py [port]   (default 8090)
+
+The site must be built first:      make build index
+Then start this:                   python3 serve_lan.py
+Address to open from ANOTHER machine on the same network:
+    http://<THIS-MACHINE-LAN-IP>:8090/
+where the LAN IP is printed at startup (and is also  hostname -I  | head -1).
+"""
+import functools
+import http.server
+import socket
+import subprocess
+import sys
+
+PORT = int(sys.argv[1]) if len(sys.argv) > 1 else 8090
+
+# Serve the built site (public/), NOT the repo root.
+handler = functools.partial(http.server.SimpleHTTPRequestHandler, directory="public")
+server = http.server.ThreadingHTTPServer(("0.0.0.0", PORT), handler)
+
+
+def lan_ip() -> str:
+    try:
+        return subprocess.check_output(["hostname", "-I"], text=True).split()[0]
+    except Exception:
+        s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+        try:
+            s.connect(("8.8.8.8", 80))
+            return s.getsockname()[0]
+        finally:
+            s.close()
+
+
+ip = lan_ip()
+print(f"Serving public/  ->  http://0.0.0.0:{PORT}/", flush=True)
+print(f"LAN address      ->  http://{ip}:{PORT}/   (open this from another machine)", flush=True)
+print("Press Ctrl+C to stop.", flush=True)
+try:
+    server.serve_forever()
+except KeyboardInterrupt:
+    pass
+finally:
+    server.server_close()