build: add preflight validator family for versions/manifest/changelog drift

Establishes the local pre-push gate. preflight.sh runs four blocks: version
consistency, manifest shape (Icon plus all ImageUrls), changelog sync, plus a
release build as compile-health smoke. setup-hooks.sh wires core.hooksPath to
.githooks. .gitignore opens scripts/ for tracking (setup-dev-env.sh stays
private). Test execution itself lives in a separate local repository and is
not part of this codebase.

.gitignore

@@ -9,7 +9,7 @@
 .envrc
 !.env.example
 .vscode/
-scripts/
+scripts/setup-dev-env.sh
 
 # Local test project (stays out of the published plugin repo;
 # pure-function safety net for refactor cycles)