docs: document admin-only governance & slnx upkeep rule

[thorstenalpers]

Documents the governance rules for this repo — what is admin-only and must never be done by an AI agent on its own.

AGENTS.md — new "Roles & permissions (admin-only actions)" section

• Reviewing & merging PRs into main is admin-only. Agents prepare branches and open PRs, then stop — no merging, no auto-merge, no merging their own PRs. The admin manually reviews every PR.
• Opening PRs automatically is allowed only while an agent is explicitly operated by the admin.
• Triggering deploy-nuget.yml (publish + tag + GitHub Release) is a manual, admin-only step, run after the admin merges the release PR.

AGENTS.md — slnx upkeep rule

Added a note (layout section + "What NOT to do") that OpenTelemetryExtension.slnx tracks loose files explicitly, so any added/renamed/deleted tracked file — including a new release-notes/v{VERSION}.md — must be reflected in the .slnx.

prepare-release skill

• Header + steps 12–14 updated: the skill opens the release PR and stops — it does not merge or publish. Step 11 now reminds to add the release note to the .slnx. Step 14 reframed so the admin merges and triggers Deploy Nuget.

Versioning & release section

Cross-references the new Roles & permissions section.

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Per the rule it documents, this PR is opened, not merged — left for admin review.