Add event-driven eBPF execve layer with a Snort-style rule engine

Closes polling's blind spot (processes that exit between sweeps) with a real
eBPF probe and a declarative, data-driven detection engine — inspired by Snort
(rule language, signature IDs) and OSSEC (host-IDS framing).

New events/ subpackage:
- bcc_source.py : eBPF C tracing execve (filename, argv[1..2], ppid, uid,
                  parent comm) over a perf buffer, loaded via bcc; lazy import
                  + available()/try-except so it fails closed to poll-only when
                  bcc/root/BTF are absent — a broken probe never downs the daemon
- exec_event.py : the ExecEvent type
- rules.py      : ExecRule (sid/msg/severity/classtype + path/exec/parent/argv
                  conditions) and ExecRuleEngine; 4 shipped rules (fileless exec
                  100001, reverse-shell argv 100002, web/DB→shell RCE 100003,
                  curl|sh 100004); operators add more via exec_rules_file TOML
- monitor.py    : runs the source on a thread, routes events through the engine

Integration:
- daemon starts the monitor, shares a lock-guarded cooldown with the sweep
  loop, and feeds event alerts into the same snapshot pipeline
- Alert gains Snort-style sid + classtype; retrofitted onto all 7 poll
  detectors; snapshots and JSON now carry them
- config: ebpf_exec_monitor (default on, degrades), exec_rules_file
- systemd: opt-in ebpf.conf drop-in (relaxes MemoryDenyWriteExecute + widens
  caps for bcc's JIT) so the base unit stays hardened for poll-only
- sentinel-redteam: ebpf_exec drill (short-lived /tmp exec + /dev/tcp argv the
  poller can't see); footer now uses the Python CLI

Tests: +14 cases for the rule engine (each default rule match/non-match, rule
validation, parent-exclude). 39/39 pass. Graceful non-root degradation verified.

NOTE: the eBPF C follows bcc's execsnoop pattern but could not be run here
(BPF needs root); it wants a root smoke-test on a real host.

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

enodia_sentinel/events/exec_event.py

@@ -0,0 +1,43 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""The execve event — one process-execution observed by the kernel probe."""
+from __future__ import annotations
+
+import os
+from dataclasses import dataclass, field
+
+
+@dataclass(frozen=True)
+class ExecEvent:
+    """A single ``execve``, as seen at the moment of execution.
+
+    Captured by the eBPF probe (so it exists even for processes that exit before
+    a poll). ``parent_comm`` is the caller; ``filename`` is the program being
+    executed; ``argv`` is as much of the argument vector as the probe captured.
+    """
+
+    pid: int
+    ppid: int
+    uid: int
+    parent_comm: str
+    filename: str
+    argv: tuple[str, ...] = field(default_factory=tuple)
+
+    @property
+    def exec_comm(self) -> str:
+        """Basename of the executed program (its post-exec ``comm``)."""
+        return os.path.basename(self.filename)
+
+    @property
+    def argv_str(self) -> str:
+        """Full command line for regex matching (filename + captured args)."""
+        return " ".join((self.filename, *self.argv)).strip()
+
+    def to_dict(self) -> dict:
+        return {
+            "pid": self.pid,
+            "ppid": self.ppid,
+            "uid": self.uid,
+            "parent_comm": self.parent_comm,
+            "filename": self.filename,
+            "argv": list(self.argv),
+        }