[Epic]: Citation integrity — ADRs and docs that resolve to one true thing

### Summary

ADR numbers are cited as cross-repo contracts. Three of them currently resolve to **two Accepted decisions each**, and a fourth family of tickets cites ADRs that do not exist at all.

### Intent

A citation that resolves to two documents is not a contract. This has already cost real investigation time and, worse, is load-bearing right now: a live in-progress ticket (#1063) depends on "ADR-0118" while two Accepted files carry that number, and six tickets touching the **auth boundary** cite ADR-0077/0078, which on `main` are Grafana dashboard and span-attribution decisions.

### Children

- **#1073** — renumber the three collisions (0050, 0077, 0118) and add a CI guard
- **#1074** — decide the fate of the GitLab epic whose ADR-0077/0078 citations point at nothing
- **#1051** — sync the cert-manager docs with the shipped Route53 DNS-01 fix

### Acceptance Criteria

1. `ls docs/adr/*.md | sed 's#.*/##' | cut -d- -f1 | sort | uniq -d` returns empty, and CI keeps it that way.
2. No open issue in any repo cites ADR-0077 or ADR-0078 for a GitLab/multi-forge claim.
3. **House rule recorded: cite ADRs by filename, not bare number**, at least until the guard exists.

### Out of Scope

- Editing any ADR's decision body — they are Accepted and immutable.

### Technical Context

⚠️ **#1073's own AC names "0136" as the next free number.** That is true today and will be wrong as soon as any ADR merges — which would instruct a fourth collision. Compute `max(prefix)+1` at PR time instead; the CI guard is the real check.

### AI Usage Declaration

Grouping identified and drafted by Claude Opus 5 during the 2026-09-01 triage sweep; the collisions and the missing GitLab ADR were verified on `origin/main`. AI output is not truth; the owner verifies.