fix: require ANTHROPIC_API_KEY for Claude Code auth in VNC container

Browser-based OAuth login does not work inside the VNC session because
the OAuth redirect callback cannot reach back into the container. The
solution is to set ANTHROPIC_API_KEY in the Kubernetes secret — when
this env var is present, Claude Code skips browser auth entirely.

Changes:
- init-repo.sh: warn clearly at startup if ANTHROPIC_API_KEY is unset
- values.yaml: document ANTHROPIC_API_KEY in the envSecretName comment
- VARIABLES.md: add ANTHROPIC_API_KEY entry and update secret template

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

chart/values.yaml

@@ -38,6 +38,9 @@ resources:
     memory: "8Gi"
     cpu: "4000m"
 
-# Name of existing Secret containing env vars (GITHUB_TOKEN, VNC_PASSWORD, etc.)
-# Defaults to: devcontainer-{name}-secrets-env
+# Name of existing Secret containing env vars. Defaults to: devcontainer-{name}-secrets-env
+# Recognized keys:
+#   GITHUB_TOKEN      — PAT for private repo access
+#   VNC_PASSWORD      — password for the VNC web UI
+#   ANTHROPIC_API_KEY — required for Claude Code / Happy Coder auth (browser login won't work in VNC)
 envSecretName: ""