docs: comprehensive documentation expansion

Add 4 new doc files (configuration reference, development guide, API
reference with full request/response schemas, database schema), expand
the README with a feature list and how-it-works walkthrough, fix
missing Firmware and Burn stages in architecture.md and test-suite.md,
add threshold engine and host-mode agent sections, and add godoc
comments to 11 packages and 6 model types.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/vetting-agent/main.go

@@ -1,3 +1,7 @@
+// Agent binary. Runs in two modes: live-image (default, no args)
+// parses /proc/cmdline and enters the claim loop; host-mode
+// ("vetting-agent host") reads /etc/vetting/host-agent.yaml and
+// becomes a persistent heartbeat reporter.
 package main
 
 import (