docs: mark admin-merge policy as transitional solo-owner state #93

Problem

CONTRIBUTING.md around lines 47–56 documents `gh pr merge --admin` as standard merge policy. In the current solo-owner phase that's correct — branch protection requires reviewers that don't exist yet — but as written, an external reader could read it as the maintainer routinely bypassing their own gates.

Proposed solution

Add a clarifying header / preamble to the admin-merge section in CONTRIBUTING.md, e.g.:

Transitional — only while this repo has a single owner. Branch protection requires at least one reviewer; until a second maintainer exists, the owner uses gh pr merge --admin after all required checks pass. Once a co-maintainer is onboarded, this exemption is removed and standard review applies.

Optionally link the corresponding note in docs/DEVELOPMENT.md if one exists there.

Acceptance criteria

• CONTRIBUTING.md admin-merge section has a "transitional" header / preamble
• Wording makes clear it's a single-owner workaround, not a bypass policy
• Explicit trigger for removing the exemption (second maintainer onboarded) is stated

Priority rationale

Avoids a wrong first impression about how the project treats its own quality gates.