chore: enforce OpenSpec format on issues and PRs

[jamubc]

Why

Issues and PRs across Jam-Sw follow the OpenSpec format, but this repository does not yet validate bodies, so malformed submissions land unflagged and reviewers discover missing sections by hand.

What Changes

• Add .github/workflows/validate.yml, the two-line caller for the org-wide reusable OpenSpec validator in Jam-Sw/.github

MODIFIED Requirements

Requirement: OpenSpec body validation

Issues and pull requests in this repository SHALL be checked for the required OpenSpec sections (Why, What Changes, MODIFIED Requirements, GIVEN/WHEN/THEN scenario, numbered Verification) when opened or edited. Bodies missing sections MUST receive a needs-info label and a comment listing what is missing, and the check MUST fail until the body is corrected. Once corrected, the label MUST be removed automatically.
(Previously: no validation ran; malformed issues and PRs were accepted silently.)

Scenario

GIVEN this workflow is merged to the default branch
WHEN an issue is opened with a body missing the Verification section
THEN the OpenSpec check fails, the issue is labeled needs-info, and a comment lists the missing section

Verification

• 1.1 Open a test issue missing a section and confirm the check fails with the needs-info label and an explanatory comment
• 1.2 Edit the issue to include all sections and confirm the check passes and the label is removed