Environment variable family migration

[JSONbored]

Problem: Roughly twenty self-host-facing environment variables, including a whole family of feature-flag variables, are prefixed with the old brand name.

Area: Self-host / Config

Proposal: Introduce equivalents under the new prefix; dual-read both prefixes (documenting which one wins if both are set); regenerate the committed environment-reference documentation last, in the same PR, after every variable change lands.

Deliverables:

• Dual-read support for every affected variable.
• Regenerated environment-reference documentation, committed in the same PR.

Acceptance criteria:

• An existing self-hoster's .env using only the old prefix keeps working unchanged.
• The generated reference-documentation drift check passes in CI.

Test scenarios:

• Unit tests covering both prefixes for a representative sample of the affected variables.
• Run the environment-reference generator and confirm no diff against the committed file.

Resources:

• The example environment files, the environment-reference generator script, and its generated output file.

Boundaries:

• Regenerate the reference documentation last, after all variable changes are in — regenerating early and then adding more variables re-breaks the generated file (a known project gotcha).

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Part of #4761.

[linear-code]

JSO-3446