feat: add tier detection package

Probes Anthropic and LiteLLM endpoints to detect the current operating
tier (Full / LANOnly / Airplane) so downstream code can gate model
selection and managed-agent availability without manual configuration.

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

internal/tier/tier.go

@@ -0,0 +1,64 @@
+// internal/tier/tier.go
+package tier
+
+import (
+	"context"
+	"net/http"
+	"time"
+)
+
+// Tier represents the current operating capability level.
+type Tier int
+
+const (
+	Full     Tier = 1 // internet + Anthropic API reachable
+	LANOnly  Tier = 2 // LiteLLM on LAN reachable, no internet
+	Airplane Tier = 3 // no network
+)
+
+// Info describes the current operating tier.
+type Info struct {
+	Tier            Tier     `json:"tier"`
+	Label           string   `json:"label"`
+	AvailableModels []string `json:"available_models"`
+	ManagedAgents   bool     `json:"managed_agents"`
+}
+
+// Detect probes the Anthropic endpoint and LiteLLM and returns the current tier.
+// Each probe has a 2-second timeout.
+func Detect(ctx context.Context, anthropicProbe, liteLLMBaseURL string) Info {
+	client := &http.Client{Timeout: 2 * time.Second}
+
+	if probe(ctx, client, anthropicProbe) {
+		return Info{
+			Tier:          Full,
+			Label:         "full-online",
+			ManagedAgents: true,
+		}
+	}
+	if probe(ctx, client, liteLLMBaseURL) {
+		return Info{
+			Tier:          LANOnly,
+			Label:         "lan-only",
+			ManagedAgents: false,
+		}
+	}
+	return Info{
+		Tier:          Airplane,
+		Label:         "airplane",
+		ManagedAgents: false,
+	}
+}
+
+func probe(ctx context.Context, client *http.Client, url string) bool {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return false
+	}
+	resp, err := client.Do(req)
+	if err != nil {
+		return false
+	}
+	resp.Body.Close()
+	return true
+}