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>

config/enodia-sentinel.toml

@@ -55,6 +55,18 @@ watch_persistence = [
     "/root/.ssh/authorized_keys", "/root/.bashrc", "/root/.profile",
 ]
 
+# --- file integrity monitoring (FIM) ------------------------------------
+# SHA-256 baseline of security-critical files the package manager doesn't track
+# (configs, /usr/local, systemd units, keys). A pacman PostTransaction hook runs
+# `enodia-sentinel fim-update` after every upgrade, so package changes never
+# alert. Package-owned binaries are instead verified against the distro's own
+# checksums via `pacman -Qkk` (fim_pkg_verify).
+fim_enabled = true
+fim_scan_interval = 300          # seconds between hash scans
+fim_paths = []                   # [] = built-in critical set; or your own list
+fim_pkg_verify = false           # run `pacman -Qkk` (thorough but slow)
+fim_pkg_verify_interval = 21600  # seconds between package verifications
+
 # --- eBPF event layer ----------------------------------------------------
 # Event-driven execve monitor: catches short-lived processes the poll loop
 # misses, matched against the Snort-style rule engine. Requires python-bpfcc