V-L3-T1: write CONTRIBUTING-DEV.adoc (how to add a backend / drift category / overlay)

[hyperpolymath]

Context

.github/CONTRIBUTING.md covers RSR conventions but not "how to add a backend / drift category / overlay" for this codebase.

What to do

Add CONTRIBUTING-DEV.adoc walking through:

• Cloning + cargo workflow
• How to add a backend (DatabaseBackend variant + interception strategy + DDL dialect)
• How to add a drift category (ADR + measure function + test)
• How to add a Tier 2 overlay (manifest field + storage + interceptor)

Acceptance

• File exists with each section having at least a checklist