Add file integrity monitoring (Tripwire-style), auto-refreshed via pacman hook

Detects binary/config tampering by content hash — catching a malicious swap
even when mtime is preserved (the gap in mtime-based persistence checks). Two
engines split by file ownership:

- fim.py hash baseline: SHA-256 (+ mode/uid/gid/size) of security-critical
  files the package manager doesn't track (/usr/local, /etc configs, systemd
  units, SSH keys). The baseline refreshes ONLY via fim-update / the pacman
  hook, so a flagged change stays flagged until acknowledged (Tripwire
  semantics). Alerts: fim_modified 100017 / fim_added 100018 / fim_removed 100019.
- package verification: `pacman -Qkk` checks package-owned binaries against the
  distro's own signed checksums — no baseline to maintain, implicitly current
  because the package DB updates on every upgrade. fim_pkg_modified 100020.

Auto-update on system updates: a pacman PostTransaction hook runs
`enodia-sentinel fim-update` after every install/upgrade/remove, so legitimate
package changes never alert — no manual `tripwire --update`.

- daemon: backgrounded FIM scan + optional pkg-verify on slow cadences, feeding
  the normal alert/snapshot/push pipeline; baseline loaded/built at startup
- cli: fim-baseline / fim-update / fim-check [--packages]
- config: fim_enabled, fim_paths, fim_scan_interval, fim_pkg_verify[_interval]
- packaging: ship + install the pacman hook (Makefile + PKGBUILD)
- tests: +7 (hashing, diff incl. mtime-preserving tamper, pacman -Qkk parse).
  72/72 pass. Verified end-to-end: a content swap with preserved mtime is caught.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/PKGBUILD

@@ -1,6 +1,6 @@
 # Maintainer: Enodia
 pkgname=enodia-sentinel
-pkgver=0.4.0
+pkgver=0.5.0
 pkgrel=1
 pkgdesc="Host intrusion-detection daemon — signature-based detection of reverse shells, LD_PRELOAD rootkits, fileless malware, and persistence tampering"
 arch=('any')
@@ -23,6 +23,7 @@ package() {
     install -Dm755 src/sentinel-redteam "$pkgdir/usr/bin/sentinel-redteam"
     install -Dm644 systemd/enodia-sentinel.service "$pkgdir/usr/lib/systemd/system/enodia-sentinel.service"
     install -Dm644 systemd/enodia-sentinel-web.service "$pkgdir/usr/lib/systemd/system/enodia-sentinel-web.service"
+    install -Dm644 packaging/enodia-sentinel-fim.hook "$pkgdir/usr/share/libalpm/hooks/enodia-sentinel-fim.hook"
     install -Dm644 config/enodia-sentinel.toml "$pkgdir/etc/enodia-sentinel.toml"
     install -Dm644 LICENSE "$pkgdir/usr/share/licenses/$pkgname/LICENSE"
     install -dm750 "$pkgdir/var/log/enodia-sentinel"