Add generic repository hygiene checks

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Summary

Adds always-on, stack-agnostic repository hygiene checks for the template.

Changes

• Adds scripts/hygiene.sh for generic repository checks
• Runs hygiene checks from scripts/validate.sh before optional project-specific checks
• Adds a Repository hygiene step to .github/workflows/reusable-quality.yml
• Documents the generic checks in docs/DEVEX.md

Generic checks

• required template files are present
• shell scripts parse successfully
• unresolved merge markers are not present
• Markdown files start with a top-level heading or front matter

Notes

This keeps language-specific format/lint/test checks configurable while making default CI useful before a downstream project chooses a stack.

Closes #14