build: git-hooks - Add video content validation #400

Build Summary

Add path-triggered read-only pre-commit validation for content/videos/**/*.md, consuming approved profile and schema-aware CLI. Cover valid/invalid fixtures, nested discovery, spaces and unrelated-file behaviour; preserve index/working tree.

Dependencies / Blockers

Existing schema and schema-aware CLI work must deliver approved runtime profile and executable command; interfaces or standalone schemas are insufficient. Do not create competing schemas.

Acceptance Criteria

• Approved runtime profile and executable validator available.
• Valid content passes; invalid content blocks commit with file-specific errors.
• Discovery/exclusions match approved content model.
• No journal-specific rules imposed.
• No migration, editorial changes, exports or auto-staging.
• Hook behaviour/recovery instructions have reproducible evidence.

Definition of Ready

• Scope, fixtures, prerequisites and overlap identified.

Definition of Done

• Criteria pass; commands, revision, environment and exit codes recorded.
• Documentation reflects tests; applicable review/checks pass.

Branch instructions

Recheck repo instructions, active work and PR template; independently reviewable branch from develop; no automatic merged-branch reuse. Preserve unrelated changes; no force-push, merge or deployment. Estimate unset; no dates/milestone.