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/config.py

@@ -61,6 +61,13 @@ class Config:
     )
     watch_persistence: tuple[str, ...] = _DEFAULT_WATCH
 
+    # file integrity monitoring (FIM)
+    fim_enabled: bool = True
+    fim_paths: tuple[str, ...] = ()          # populated from fim.DEFAULT_FIM_PATHS
+    fim_scan_interval: int = 300             # seconds between hash scans
+    fim_pkg_verify: bool = False             # run `pacman -Qkk` (slow; opt-in)
+    fim_pkg_verify_interval: int = 21600     # seconds between package verifications
+
     # eBPF on-ramp
     capture_execve_bpftrace: bool = False
     # Event-driven eBPF execve monitor (catches short-lived processes the poll
@@ -107,6 +114,17 @@ class Config:
     def suid_baseline(self) -> Path:
         return self.log_dir / "suid-baseline.json"
 
+    @property
+    def fim_baseline(self) -> Path:
+        return self.log_dir / "fim-baseline.json"
+
+    def fim_path_list(self) -> tuple[str, ...]:
+        """Configured FIM paths, defaulting to the built-in critical set."""
+        if self.fim_paths:
+            return self.fim_paths
+        from .fim import DEFAULT_FIM_PATHS
+        return DEFAULT_FIM_PATHS
+
     # ---- loading ---------------------------------------------------------
     @classmethod
     def load(cls, path: str | os.PathLike | None = None) -> "Config":