docs: decision note 0053, no merge on a red check and evidence for every real test

[marcos-mendez]

Records the two rules the maintainer decided on 2026-10-08, asked what would make the project enterprise-grade and trustworthy.

• No pull request merges with a red check, required or not; a red check is a bug fixed before the merge, and a wrong check is fixed in the first PR. Removing a required context to merge ends. The case: keel-mariadb's appliance / boot-published-layer, red since 2026-09-30, fixed by reordering tests/lib/boot-test-lib.sh so the primary converges before the replica's refusal step.
• Every test on the real platform leaves evidence/<date>-<topic>.md in the PR or tracker issue: commands, output trimmed and never paraphrased, dpkg versions, artifact hashes, timings. A report without it is not a result. The case: the mesh nodes reported at 0.22.0 from dpkg and etcd checks while keel --version printed 0.19.0 (keel#98); with the file the mismatch is on adjacent lines.
• Lists under "What comes next", undecided, the other items heard the same day: support lines, live pair upgrade as a release criterion (keel#91), security process, pair restore in CI, metrics and logs, runbooks, governance, fewer appliances first.
• Adds 0053 to the index.