v0.35.0: bare extends: claude mixin fails compose with "credential for service anthropic defined in both"

[clanchorman]

Summary

Upgrading 0.34.0 → 0.35.0, creating a sandbox from a mixin that does nothing but
extends: claude fails at kit artifact composition with a duplicate-credential error
for the base agent's built-in anthropic credential. The mixin declares no
credentials, services, or domains of its own. Reverting to 0.34.0 fixes it.

Environment

• sbx 0.35.0 — fails
• sbx 0.34.0 — works (clean bisect)

Minimal reproduction

spec.yaml (complete — nothing omitted):

schemaVersion: "1"
kind: mixin
name: sbx-035-bug
extends: claude
displayName: Repro

Create a sandbox using this mixin.

Actual result

ERROR: request failed: 400 Bad Request:
kit_artifacts: compose: compose: credential for service "anthropic"
defined in both "claude" and "sbx-035-bug"

Expected result

Composition succeeds. The anthropic credential is owned by the base claude
agent and only reaches the child layer via extends; it should be deduped, not
counted as two definitions.

Notes / ruling out the usual causes

• Not a stale/duplicate secret — sbx secret list shows a single anthropic
  (OAuth), no api-key shadow:

  (global)  service  anthropic  (oauth configured)
  

• The mixin adds no network.serviceDomains / credentials, so the content
  itself cannot introduce a second anthropic. The duplicate is produced purely
  by the extends merge.