Add missing label for Easy Mode GitHub

`tests/test_credential_consumers.py::test_every_credential_pointer_knob_has_a_label_and_no_label_is_stale` fails on develop: the operator-settings knob `easy_mode_github_credential_id` (declared in `apps/api/app/operator_settings/catalog_billing.py` around line 232, with a `credential_type`) has no entry in `KNOB_LABELS` in `apps/api/app/credential_consumers.py`, so the credential consumers roster cannot name it and the label census asserts. Found while running changed-area API tests for Task 13569, whose diff touches neither file (both identical to develop). Add the one label (next to the existing "Easy Mode runner app" entry) in the wording the Easy Mode feature uses for that credential, and confirm the consumers roster shows it for a credential bound to that knob. Out of scope: any other knob, any change to the catalog itself, or to how consumers are rendered. Done means: - `./dc test -q tests/test_credential_consumers.py` passes. - The new label reads as the Easy Mode feature names that credential (one line, no new vocabulary). Required checks: ./dc check quick

Already shipped: the knob already has its label, "Easy Mode GitHub admin", at credential_consumers.py:143, added by Task 13628 (006c2aab60). The consumers test file passes on develop (26 passed). No PR.