Re-architect Sentinel as a zero-dependency Python package

Port the bash prototype to a structured, testable Python codebase while
preserving the same control loop and the seven detection signatures. The bash
implementation stays in src/ as the regression oracle.

Highlights:
- enodia_sentinel/ package (stdlib only — no runtime deps):
  - detectors/ : one pure function per signature, detect(state, cfg)->Alerts
  - system.py  : SystemState — one cached /proc + ss snapshot per sweep,
                 fully injectable so detectors are unit-testable
  - daemon.py  : sweep loop, in-process cooldown dedup, threaded snapshot
                 capture, and a SUID filesystem scan moved OFF the loop
                 thread onto a slow background cadence
  - snapshot.py: forensic text + JSON sidecar with per-signature IR guidance
  - config.py  : dataclass config via TOML + env overrides
  - netutil.py : public-IP / CIDR logic via stdlib ipaddress
- tests/ : 25 stdlib-unittest cases (no root, no /proc, no ss needed)
- TOML config, launcher wrapper, Makefile (pip-free install), hardened
  systemd unit (env-resolved ExecStart), updated PKGBUILD, rewritten README

Performance: per-sweep cost ~200 ms (shared cached state); the multi-second
SUID walk no longer blocks detection. scandir-based walk replaces os.walk.

Verified on Arch: all 7 detectors fire on red-team drills (reverse_shell,
ld_preload, deleted_exe, new_listener, new_suid confirmed live end-to-end),
no false positives on a clean sweep, 25/25 tests pass.

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

systemd/enodia-sentinel.service

@@ -5,24 +5,29 @@ Documentation=https://github.com/Enodia/enodia-sentinel
 
 [Service]
 Type=simple
-ExecStart=/usr/local/bin/sentinel.sh
+# /usr/bin/env resolves the launcher from PATH, so this works whether it was
+# installed under /usr (package) or /usr/local (make install).
+ExecStart=/usr/bin/env enodia-sentinel run
 Restart=on-failure
 RestartSec=5
 
-# Needs to read every process's /proc, sockets, and SUID inventory, so it runs
-# as root — but with the surface area locked down. It only ever READS the system
-# and WRITES to its own log dir.
+# It only ever READS the system and WRITES to its own log dir. Lock the rest
+# of the surface down: needs root to read every /proc and root-owned files,
+# but with a minimal capability set and no ability to modify the system.
 ProtectSystem=strict
 ReadWritePaths=/var/log/enodia-sentinel
 ProtectHome=read-only
 NoNewPrivileges=yes
 ProtectKernelTunables=yes
+ProtectKernelModules=yes
 ProtectControlGroups=yes
 RestrictSUIDSGID=yes
 MemoryDenyWriteExecute=yes
 LockPersonality=yes
+RestrictNamespaces=yes
 # CAP_SYS_PTRACE: read other processes' /proc; CAP_DAC_READ_SEARCH: read
 # root-owned files (authorized_keys, ld.so.preload) regardless of perms.
+CapabilityBoundingSet=CAP_SYS_PTRACE CAP_DAC_READ_SEARCH
 AmbientCapabilities=CAP_SYS_PTRACE CAP_DAC_READ_SEARCH
 
 [Install]