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>

enodia_sentinel/cli.py

@@ -63,6 +63,11 @@ def main(argv: list[str] | None = None) -> int:
     sub.add_parser("list-detectors", help="list available detectors")
     sub.add_parser("web", help="serve the read-only dashboard")
     sub.add_parser("triage", help="classify captured alerts as likely-FP vs review")
+    sub.add_parser("fim-baseline", help="build the file-integrity baseline")
+    sub.add_parser("fim-update", help="refresh the FIM baseline (run by the pacman hook)")
+    fc = sub.add_parser("fim-check", help="scan monitored files and report changes")
+    fc.add_argument("--packages", action="store_true",
+                    help="also verify package-owned files via pacman -Qkk")
 
     args = parser.parse_args(argv)
     cfg = Config.load(args.config)
@@ -82,9 +87,40 @@ def main(argv: list[str] | None = None) -> int:
         return 0
     if args.cmd == "triage":
         return _cmd_triage(cfg)
+    if args.cmd in ("fim-baseline", "fim-update"):
+        n = Sentinel(cfg).build_fim_baseline()
+        print(f"FIM baseline written: {n} files under {cfg.log_dir}")
+        return 0
+    if args.cmd == "fim-check":
+        return _cmd_fim_check(cfg, args.packages)
     return _cmd_run(cfg)
 
 
+def _cmd_fim_check(cfg: Config, packages: bool) -> int:
+    from . import fim
+    sentinel = Sentinel(cfg)
+    sentinel.load_fim_baseline()
+    current = fim.scan_paths(cfg.fim_path_list())
+    d = fim.diff(sentinel.fim_baseline, current)
+    changed = len(d["added"]) + len(d["removed"]) + len(d["modified"])
+    for path, changes in d["modified"]:
+        print(f"[MODIFIED] {path}  ({', '.join(changes)})")
+    for path in d["added"]:
+        print(f"[ADDED]    {path}")
+    for path in d["removed"]:
+        print(f"[REMOVED]  {path}")
+    if not changed:
+        print("FIM: no changes against baseline.")
+    if packages:
+        print("\nVerifying package-owned files (pacman -Qkk, may take a while)…")
+        hits = fim.pacman_verify()
+        for pkg, path, reason in hits:
+            print(f"[PKG]      {path}  ({pkg}: {reason})")
+        if not hits:
+            print("Packages: all verified files match the distro checksums.")
+    return 1 if changed else 0
+
+
 def _cmd_triage(cfg: Config) -> int:
     import json
     from collections import OrderedDict