docs: ADR for squash-only merge policy + org transfer to metal-up-your-ass

[yves-vogl]

Context

Two repo-level decisions from 2026-07-14 are not yet documented as ADRs (the repo already uses ADR-0001/0002):

1. Squash-only merge policy. main enforces required_linear_history (classic protection) plus a "Signed commits on main" ruleset (required_signatures). Together these make merge commits unusable (linear history) and rebase-merges unusable (GitHub does not sign rebased commits → ruleset rejects them), leaving squash as the only working merge method. Consequence: squash commits on main are authored as GitHub-noreply + GitHub-signed, while local commits stay yves.vogl@mac.com + GPG. Accepted as the standing policy.
2. Org transfer. Repo moved from personal yves-vogl/twist-your-guts to org metal-up-your-ass/twist-your-guts. No new branch-protection / ruleset / Actions constraints were added (verified 0 approvals, no CODEOWNERS, allowed_actions: all).

Task

Add ADR(s) under docs/adr/ documenting both decisions with rationale + consequences, following the existing ADR format.

Acceptance criteria

• ADR(s) committed under docs/adr/.
• Merge policy + authorship consequence documented.
• Org transfer + governance state documented.