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/deleted_exe.py

@@ -0,0 +1,39 @@
+"""deleted_exe — a process running from a deleted or memfd-backed binary.
+
+Fileless malware unlinks its dropper (or runs straight from ``memfd``) so there
+is nothing on disk to scan. The kernel still labels the ``/proc/<pid>/exe``
+symlink ``(deleted)`` or ``memfd:``. Normal-path deleted exes (a daemon still
+running after a package upgrade) are ignored — only attacker-controlled/fileless
+locations alert.
+"""
+from __future__ import annotations
+
+from collections.abc import Iterator
+
+from ..alert import Alert, Severity
+from ..config import Config
+from ..system import SystemState
+
+_SUSPICIOUS_PREFIXES = ("/tmp/", "/dev/shm/", "/var/tmp/", "/run/")
+
+
+def _is_fileless(exe: str) -> bool:
+    if "memfd:" in exe:
+        return True
+    if "(deleted)" not in exe:
+        return False
+    return exe.startswith(_SUSPICIOUS_PREFIXES)
+
+
+def detect(state: SystemState, cfg: Config) -> Iterator[Alert]:
+    for proc in state.processes:
+        exe = proc.exe
+        if not exe or not _is_fileless(exe):
+            continue
+        yield Alert(
+            severity=Severity.CRITICAL,
+            signature="deleted_exe",
+            key=f"del:{proc.pid}",
+            detail=f"pid={proc.pid} comm={proc.comm} exe=[{exe}]",
+            pids=(proc.pid,),
+        )