Separate before serializing shared state

Remove the shared write before coordinating it. This lens is for code and tooling design; it does not change how agent work is divided.

• Before actors share state, ask what a concurrent modification would do; isolate the state unless the answer is nothing.
• By default, give each concurrent writer its own write target (a file, key, branch or directory) and merge only where the results are read.
• Where one shared target is a real invariant, serialize access by structure (a lock, a sequential phase, a single writer, compare-and-swap), never by an instruction to take turns.