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/events/events.go

@@ -1,3 +1,6 @@
+// Package events provides an in-process SSE fan-out hub. Browser
+// clients subscribe via GET /events; the orchestrator publishes
+// pre-rendered HTML fragments that HTMX swaps into the DOM.
 package events
 
 import (