fix(planning): salvage archived change links to flat 2.0.0 files

Pre-existing changes/archive/<slug>/ (and active/) dead links repointed to changes/<slug>.md where the flat file exists at top level. Docs-only.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lesnik512

planning/changes/2026-06-13.01-portable-planning-convention.md

@@ -149,7 +149,7 @@ no invented PR links.
 ### 6. Dogfood
 
 This change is its own bundle:
-`planning/changes/active/2026-06-13.01-portable-planning-convention/`. On merge
+`planning/changes/2026-06-13.01-portable-planning-convention.md`. On merge
 it moves to `archive/` with `status: shipped`, `pr:`, and `outcome:` filled, and
 its Index line shifts from Active to Archived. No `architecture/` promotion —
 this change defines the convention, not a library capability.