Group branches.yaml by EL/CL/MEV/ZK/Tooling/Other/Lean

Restructures branches.yaml so each top-level key is a group whose
`common_branches:` list is merged into every client in that group, with
`skip_common: true` as a per-client opt-out. Trims repeated devnet branch
lists for ELs/CLs while leaving the generated config.yaml byte-identical.

- el.common_branches: bal-devnet-3, bal-devnet-7, glamsterdam-devnet-4
  (opted out: ethereumjs)
- cl.common_branches: glamsterdam-devnet-4
  (opted out: caplin, consensoor)
- generate_config.py: new flatten_groups() merges common_branches per
  group and honors skip_common before the existing expansion logic runs.

Author: barnabasbusa