Wire existing validation into local pre-commit hooks #13

Describe the change

Let organization-default contributors run the existing validation before committing, with meaningful checks of the staged content rather than a pass over unrelated committed files. The repository has no .pre-commit-config.yaml; MSXOrg/.github#12 intentionally delivers only the separately approved read-only PR check for #11. The MSX Pre-Commit standard requires local hooks for repositories holding code, so local onboarding remains a captured standards exception.

This issue records a separate follow-up, not authorization for a repository-wide language/tooling rollout, live ruleset changes, or hook writes into another worktree's checkout. An owner must confirm the bounded hook scope and safe installation mechanism before implementation begins.

Acceptance criteria

• A contributor can run the existing applicable checks before a commit and receives a nonzero result for deliberately malformed staged Markdown or fallback content.
• Staged changes are actually examined; a clean committed HEAD, unrelated fixtures, or no applicable input cannot disguise unvalidated staged content.
• Local and hosted checks use the same approved shared rule configuration, with explicit version/tool setup and no duplicated formatting rules.
• Installing a hook does not silently change another app-managed worktree or the main checkout.
• Valid and malformed anonymous fixtures demonstrate the rejection and correction paths, and direct contributor documentation explains setup and the exact local command.

Technical decisions

Reuse the source-bound validator, Pester suite, and configurations under .github/scripts, .github/tests, and .github/linters. Keep the organization-defaults scope and the existing read-only hosted workflow; do not add release tooling, consumer rollout, label mutation, or live enforcement. The existing Test-OrganizationDefaults.ps1 compares committed revisions, so the staged-input adapter and per-worktree installation approach need explicit owner review before this Task is ready.

Implementation plan

• Resolve the hook scope and safe isolated-checkout/worktree installation design with the owner.
• Define failing anonymous staged-versus-committed and malformed-versus-valid tests before changing hook behavior.
• Add the minimum .pre-commit-config.yaml and staged-input wiring using the existing approved tooling.
• Document setup and exact commands without changing unrelated organization-fallback guidance.
• Verify the staged rejection/correction cases, hosted parity, and complete current-head review in one separately scoped pull request.