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>

enodia_sentinel/detectors/egress.py

@@ -0,0 +1,35 @@
+"""egress — an interpreter holding an outbound connection to a public IP.
+
+C2 beacons and exfil phone home. A scripting interpreter (bash/python/perl/nc…)
+with an established connection to a globally-routable address — that isn't in
+the operator's trust list — is worth a look.
+"""
+from __future__ import annotations
+
+from collections.abc import Iterator
+
+from ..alert import Alert, Severity
+from ..config import Config
+from ..netutil import ip_in_cidrs, is_public_ip, split_host_port
+from ..system import SystemState
+
+
+def detect(state: SystemState, cfg: Config) -> Iterator[Alert]:
+    for s in state.established_sockets():
+        if s.comm not in cfg.interpreters:
+            continue
+        host, port = split_host_port(s.peer)
+        if not is_public_ip(host):
+            continue
+        if ip_in_cidrs(host, cfg.egress_allow_cidrs):
+            continue
+        yield Alert(
+            severity=Severity.HIGH,
+            signature="egress",
+            key=f"egr:{s.pid}:{host}",
+            detail=(
+                f"pid={s.pid} comm={s.comm} -> {host}:{port} "
+                "(interpreter to public IP)"
+            ),
+            pids=(s.pid,) if s.pid else (),
+        )