add decisions-validate pre-push gate

One new Makefile target (tools/scripts/plan decisions validate)
wired as a prerequisite of push-check so malformed records fail
locally before they reach a reviewer. Parser dry-run is cheap
(~100ms) and stays within the <90s pre-push budget.

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -50,6 +50,9 @@ heading, and (b) bumping `project.yaml` `version:` in the same commit.
   [`D-040`](decisions/process.md#d-040-python-stopgap-under-toolsscriptsplan)
   / [`R-011`](decisions/rejected.md#r-011-permanent-stopgap).
 
+- `make decisions-validate` — cheap parser dry-run wired into
+  `push-check`. Catches malformed records before push.
+
 ### Removed
 
 - `docs/ADRs/` directory — content lifted into `decisions/` as D/R