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/reverse_shell.py

@@ -0,0 +1,37 @@
+"""reverse_shell — an interpreter with a network socket on its stdio.
+
+A real reverse shell dups a TCP/UDP socket onto fd 0/1/2 (``nc -e /bin/bash``,
+``bash -i >& /dev/tcp/...``). Interactive shells get a pty, and daemons get
+unix sockets/pipes — neither is a network socket, so requiring the stdio socket
+to appear in the network socket table is what keeps false positives near zero.
+"""
+from __future__ import annotations
+
+from collections.abc import Iterator
+
+from ..alert import Alert, Severity
+from ..config import Config
+from ..system import SystemState
+
+
+def detect(state: SystemState, cfg: Config) -> Iterator[Alert]:
+    peer_by_inode = state.net_peer_by_inode
+    for proc in state.processes:
+        if proc.comm not in cfg.interpreters:
+            continue
+        inode = proc.stdio_socket_inode()
+        if inode is None:
+            continue
+        peer = peer_by_inode.get(inode)
+        if not peer:  # unix socket / pipe — benign
+            continue
+        yield Alert(
+            severity=Severity.CRITICAL,
+            signature="reverse_shell",
+            key=f"rsh:{proc.pid}",
+            detail=(
+                f"pid={proc.pid} comm={proc.comm} stdio=net-socket "
+                f"peer=[{peer}] cmd=[{proc.cmdline[:90]}]"
+            ),
+            pids=(proc.pid,),
+        )