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/model/model.go

@@ -1,7 +1,11 @@
+// Package model defines the domain value types shared across the
+// orchestrator: Host, Run, Stage, SubStep, Measurement, and SpecDiff.
+// These are plain structs with no behaviour beyond state classification.
 package model
 
 import "time"
 
+// Host is a registered hardware node in the vetting cluster.
 type Host struct {
 	ID               int64
 	Name             string
@@ -17,6 +21,7 @@ type Host struct {
 	LastSeenAt       *time.Time // host-mode agent heartbeat; nil = never seen
 }
 
+// RunState is the current position of a run in the state machine.
 type RunState string
 
 const (
@@ -51,6 +56,7 @@ func (s RunState) IsTerminal() bool {
 	return false
 }
 
+// Run is a single vetting pass on a host, walking through the stage pipeline.
 type Run struct {
 	ID                int64
 	HostID            int64
@@ -68,6 +74,7 @@ type Run struct {
 	Profile           string // quick|deep|soak; empty is treated as "quick"
 }
 
+// StageState tracks whether a stage is pending, running, passed, failed, or skipped.
 type StageState string
 
 const (
@@ -78,6 +85,7 @@ const (
 	StageSkipped StageState = "skipped"
 )
 
+// Stage is a single test step within a run (e.g. SMART, CPUStress, Storage).
 type Stage struct {
 	ID          int64
 	RunID       int64
@@ -107,6 +115,7 @@ type SubStep struct {
 	SummaryJSON string
 }
 
+// Measurement is a single time-series sample from the thermal sidecar or a stage executor.
 type Measurement struct {
 	ID      int64
 	RunID   int64
@@ -118,6 +127,7 @@ type Measurement struct {
 	Unit    string
 }
 
+// SpecDiff records a single expected-vs-actual hardware divergence from SpecValidate.
 type SpecDiff struct {
 	ID       int64
 	RunID    int64