feat(compose): add client-specific profiles instead of env switch

[Sreejit40765940]

Why: README shows CLIENT=reth docker compose up; profiles make this first-class UX. GitHub

Files: docker-compose.yml Accept: docker compose --profile reth up runs only reth stack; same for geth/nethermind.

Patch (structure idea):

services: reth: profiles: ["reth"] # ... existing reth config geth: profiles: ["geth"] # ... existing geth config nethermind: profiles: ["nethermind"] # ... existing nethermind config