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/alert.py

@@ -29,11 +29,18 @@ class Alert:
     key: str
     detail: str
     pids: tuple[int, ...] = field(default_factory=tuple)
+    # Snort-style metadata: a stable signature id and a category. Every
+    # detection in Enodia has an `sid` so it can be referenced, tuned, and
+    # tracked across revisions the way a Snort/Suricata rule is.
+    sid: int = 0
+    classtype: str = "uncategorized"
 
     def to_dict(self) -> dict:
         return {
+            "sid": self.sid,
             "severity": str(self.severity),
             "signature": self.signature,
+            "classtype": self.classtype,
             "key": self.key,
             "detail": self.detail,
             "pids": list(self.pids),