docs: restructure deployment model to per-environment branches

Per-environment deployment branches replace the per-release cascade:
each environment (testnet, mainnet, scratch) gets its own short-lived
branch cut from main and FF-merged back. Testnet is now treated as
staging — iteration happens on local network or scratch deployments,
not on testnet. Rename feature/* to feat/*. Tagging required for
testnet/mainnet only; optional elsewhere.

Author: RembrandtK