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>

README.md

@@ -74,6 +74,9 @@ enodia_sentinel/
 ├── alert.py          Alert / Severity (with Snort-style sid + classtype)
 ├── web.py            read-only dashboard: stdlib http server + JSON API + auth
 ├── static/           the self-contained dashboard SPA
+├── fim.py            file integrity monitoring (hash baseline + pacman verify)
+├── triage.py         false-positive classification
+├── provenance.py     package-ownership lookups (pacman/dpkg/rpm)
 ├── detectors/        poll detectors — one module per signature, each a pure
 │                       function: detect(state, cfg) -> Iterable[Alert]
 ├── notify/           outbound push — ntfy / Pushover / webhook backends
@@ -236,6 +239,33 @@ notify_ntfy_url   = "https://ntfy.sh"
 notify_ntfy_topic = "enodia-7Hq2x"      # keep this secret — it's the access control
 ```
 
+## File integrity monitoring (Tripwire-style)
+
+Detects tampering with binaries and critical configs by **content hash**, so it
+catches a malicious swap even when the timestamp is preserved (the weakness of
+mtime-based checks). Two engines, split by who owns the file:
+
+- **Package verification** — `pacman -Qkk` checks package-owned binaries against
+  the distro's *own signed checksums*. No baseline to maintain, and it's
+  implicitly current because the package DB updates on every `pacman -Syu`. A
+  trojaned `/usr/bin/ssh` surfaces as a checksum mismatch (CRITICAL, `sid 100020`).
+- **Hash baseline** — a SHA-256 baseline of the files pacman *doesn't* track
+  (`/usr/local`, `/etc` configs, systemd units, SSH keys). A pacman
+  `PostTransaction` **hook** runs `fim-update` after every upgrade, so legitimate
+  package changes never alert — no manual `tripwire --update` ritual.
+
+```bash
+enodia-sentinel fim-baseline          # establish the baseline
+enodia-sentinel fim-check             # report changes vs baseline
+enodia-sentinel fim-check --packages  # also verify package-owned files
+```
+
+The baseline is **only** refreshed by `fim-update` / the pacman hook — a flagged
+change stays flagged until you acknowledge it, exactly like Tripwire. The daemon
+runs the scan on a slow background cadence (`fim_scan_interval`) and routes any
+change into the normal alert/snapshot/push pipeline (`fim_modified` `sid 100017`,
+`fim_added` `100018`, `fim_removed` `100019`).
+
 ## False positives & triage
 
 A host IDS that cries wolf gets ignored, so Sentinel ships explicit tooling to
@@ -319,6 +349,10 @@ regression suite for both.
 
 ## Project status
 
+v0.5 — adds **file integrity monitoring** (SHA-256 baseline + `pacman -Qkk`
+verification, auto-refreshed by a pacman hook) and **false-positive triage**
+via package-ownership provenance.
+
 v0.4 — adds a read-only **web dashboard** (stdlib server, Tailscale-bound,
 token-auth) and **phone push** (ntfy / Pushover / webhook), both zero-dependency.