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>

internal/db/db.go

@@ -1,3 +1,6 @@
+// Package db opens the SQLite database and applies embedded SQL
+// migrations in filename order at startup. Uses modernc.org/sqlite
+// (pure Go, no cgo).
 package db
 
 import (