feat(06-lab-advisor-02): add InventoryContextBuilder with 60s cache

- internal/advisor/context.go: BuildContext assembles compact NetBox
  summary (item count, category breakdown, recent 20 items)
- Caches result under sync.Mutex for defaultTTL=60s
- Stale cache returned on NetBox error rather than propagating failure
- internal/ai/types.go: add Tier3 TierConfig field to AIConfig
  (required by AdvisorHandler for OpenRouter Claude Opus access)

internal/advisor/context.go

@@ -0,0 +1,135 @@
+package advisor
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"sync"
+	"time"
+
+	"git.georgsen.dk/hwlab/internal/netbox"
+)
+
+const (
+	defaultTTL       = 60 * time.Second
+	maxContextChars  = 2000
+	maxRecentItems   = 20
+	listDevicesLimit = 200
+)
+
+// InventoryContextBuilder assembles a compact NetBox inventory summary for use as
+// the system prompt context in advisor chat. Results are cached for ttl duration
+// (default 60s) to avoid hammering NetBox on every message.
+type InventoryContextBuilder struct {
+	nb       *netbox.Client
+	mu       sync.Mutex
+	cached   string
+	cachedAt time.Time
+	ttl      time.Duration
+}
+
+// NewInventoryContextBuilder creates an InventoryContextBuilder with the default
+// 60-second cache TTL.
+func NewInventoryContextBuilder(nb *netbox.Client) *InventoryContextBuilder {
+	return &InventoryContextBuilder{
+		nb:  nb,
+		ttl: defaultTTL,
+	}
+}
+
+// BuildContext returns a compact text summary of the current NetBox inventory.
+// The result is cached for ttl to reduce NetBox API load. Thread-safe.
+//
+// Format:
+//
+//	Inventory: N items total
+//	Categories: category1 (N), category2 (N), ...
+//	Recent items:
+//	- HW-ID name (category)
+//	- ...
+func (b *InventoryContextBuilder) BuildContext(ctx context.Context) (string, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	if b.cached != "" && time.Since(b.cachedAt) < b.ttl {
+		return b.cached, nil
+	}
+
+	devices, err := b.nb.ListDevices(ctx, listDevicesLimit)
+	if err != nil {
+		// If the cache has a stale entry, return it rather than failing the request.
+		if b.cached != "" {
+			return b.cached, nil
+		}
+		return "", fmt.Errorf("advisor: build inventory context: %w", err)
+	}
+
+	summary := buildSummary(devices)
+	b.cached = summary
+	b.cachedAt = time.Now()
+
+	return summary, nil
+}
+
+// buildSummary constructs the compact text summary from a slice of devices.
+// Extracted for testability.
+func buildSummary(devices []netbox.Device) string {
+	if len(devices) == 0 {
+		return "Inventory: 0 items total\n"
+	}
+
+	// Count by category using CustomFields.HWID as asset tag source.
+	catCounts := make(map[string]int)
+	for _, d := range devices {
+		cat := categoryOf(d)
+		catCounts[cat]++
+	}
+
+	var sb strings.Builder
+	fmt.Fprintf(&sb, "Inventory: %d items total\n", len(devices))
+
+	// Categories line
+	sb.WriteString("Categories: ")
+	first := true
+	for cat, n := range catCounts {
+		if !first {
+			sb.WriteString(", ")
+		}
+		fmt.Fprintf(&sb, "%s (%d)", cat, n)
+		first = false
+	}
+	sb.WriteString("\nRecent items:\n")
+
+	limit := maxRecentItems
+	if len(devices) < limit {
+		limit = len(devices)
+	}
+	for _, d := range devices[:limit] {
+		cat := categoryOf(d)
+		assetTag := d.AssetTag
+		if assetTag == "" {
+			assetTag = d.CustomFields.HWID
+		}
+		line := fmt.Sprintf("- %s %s (%s)\n", assetTag, d.Name, cat)
+		// Guard against exceeding maxContextChars.
+		if sb.Len()+len(line) > maxContextChars {
+			sb.WriteString("- ...(truncated)\n")
+			break
+		}
+		sb.WriteString(line)
+	}
+
+	return sb.String()
+}
+
+// categoryOf returns the category label for a device, falling back to "unknown".
+func categoryOf(d netbox.Device) string {
+	// CustomFields has no Category field; fall back to checking AINotes patterns
+	// or just use "device" as default category.
+	// The catalog_status field is the closest we have to category grouping.
+	status := d.CustomFields.CatalogStatus
+	if status == "" {
+		return "device"
+	}
+	return status
+}

internal/ai/types.go

@@ -32,6 +32,7 @@ type TierConfig struct {
 type AIConfig struct {
 	Tier1               TierConfig `json:"tier1"                mapstructure:"tier1"`
 	Tier2               TierConfig `json:"tier2"                mapstructure:"tier2"`
+	Tier3               TierConfig `json:"tier3"                mapstructure:"tier3"`
 	ConfidenceThreshold float64    `json:"confidence_threshold" mapstructure:"confidence_threshold"`
 	QuickAddEnabled     bool       `json:"quick_add_enabled"    mapstructure:"quick_add_enabled"`
 	QuickAddThreshold   float64    `json:"quick_add_threshold"  mapstructure:"quick_add_threshold"`