Self-host config filename migration

[JSONbored]

Problem: The canonical self-host configuration filename carries the old brand name, and every existing self-hoster has this file committed in their own repo today.

Area: Self-host / Config

Proposal: Add the new brand's filename variants as canonical, while continuing to read the legacy filename indefinitely (or for a long, explicitly documented window). This is a dual-read addition, never a hard cutover.

Deliverables:

• Dual-read support for both old and new config filenames.
• Documentation updated to present the new filename as canonical while noting the old one still works.

Acceptance criteria:

• An existing self-hoster's untouched legacy config file continues working with zero changes required on their end.
• A new self-hoster following updated docs succeeds using only the new filename.

Test scenarios:

• Unit test with only the legacy filename present.
• Unit test with only the new filename present.
• Unit test with both present, confirming documented precedence.

Resources:

• The self-host config loader's basename list, and the equivalent public-repo config loader.

Boundaries:

• Never remove legacy filename support as part of this rebrand — that would be a separate, explicitly scheduled deprecation, not part of this milestone.

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Part of #4761.

[linear-code]

JSO-3445