fix: remove placeholder link in ADR template that breaks lychee

Replace the `[ADR-NNN](NNN-title.md)` markdown links with plain text
so the CI link checker does not try to resolve the placeholder path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Paul Mulligan

docs/adr/000-template.md

@@ -2,7 +2,7 @@
 
 ## Status
 
-[Proposed | Accepted | Deprecated | Superseded by [ADR-NNN](NNN-title.md)]
+[Proposed | Accepted | Deprecated | Superseded by ADR-NNN]
 
 ## Date
 
@@ -48,4 +48,4 @@ To propose a new ADR:
 To supersede an ADR:
 
 1. Create a new ADR referencing the old one.
-2. Update the old ADR's status to **Superseded by [ADR-NNN](NNN-title.md)**.
+2. Update the old ADR's status to **Superseded by ADR-NNN** (link to the new ADR).