fix(edit): add per-file lock to prevent read-before-write race

[spoons-and-mirrors]

what the title says. didn't wrap write.ts and patch.ts with withLock because it doesn't really make sense, but... should I for consistency?

This fixes the read before edit error we got when doing parallel edit calls (reliably?)

Resolves https://github.com/sst/opencode/issues/2882

[rekram1-node]

/review