[Branch Policy] Create and adopt main-dev as development branch

## Problem
Default branch is  and  branch does not exist, but team development policy requires .

## Required actions
1. Create  from .
2. Protect  with CI-required checks.
3. Route development PRs to .
4. Keep release merge path from  to .

## No technical debt policy
- No bypass of branch protection.
- No temporary branch alias hacks.
- Keep CI gates strict.