Add platform docs; reposition as host security platform

Add docs/ (SPECIFICATION, ROADMAP, OPERATIONS, COMMAND_REFERENCE,
THREAT_MODEL) describing the current v0.7 agent and the phased path from
local HIDS to a host security platform. Reframe README, package metadata,
and CLI/module descriptions from "intrusion-detection daemon" to "Linux
host security platform", and surface the v0.7 signed-package verify
(pkgdb_pkgverify*) and anti-rootkit (rootcheck_*) knobs in the sample
config. Also wrap the pacman.conf read in pkgdb.siglevel_alert in a
context manager to avoid a leaked file handle.

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

enodia_sentinel/pkgdb.py

@@ -169,7 +169,8 @@ def siglevel_insecure(value: str) -> bool:
 
 def siglevel_alert(conf_path: str = PACMAN_CONF) -> Alert | None:
     try:
-        text = open(conf_path, encoding="utf-8", errors="replace").read()
+        with open(conf_path, encoding="utf-8", errors="replace") as fh:
+            text = fh.read()
     except OSError:
         return None
     value = parse_global_siglevel(text)