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/cli.py

@@ -51,7 +51,7 @@ def _cmd_check(cfg: Config) -> int:
 def main(argv: list[str] | None = None) -> int:
     parser = argparse.ArgumentParser(
         prog="enodia-sentinel",
-        description="Host intrusion-detection daemon.",
+        description="Linux host security platform.",
     )
     parser.add_argument("--version", action="version",
                         version=f"enodia-sentinel {__version__}")