* feat(blocks): add Credential block
* fix(blocks): explicit workspaceId guard in credential handler, clarify hasOAuthSelection
* feat(credential): add list operation with type/provider filters
* feat(credential): restrict to OAuth only, remove env vars and service accounts
* docs(credential): update screenshots
* fix(credential): remove stale isServiceAccount dep from overlayContent memo
* fix(credential): filter to oauth-only in handleComboboxChange matchedCred lookup
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