SyncNow never locks the whole project: many agent sessions, from different team members and PCs, can be open and writing at once, and agents working on different files do not wait for each other.
Safety comes from revision-checked writes. To change an existing file, an agent must base its change on the current revision, read in its own session or named with if_match; if a colleague's agent changed the file in the meantime, the write is refused with STALE_CONTEXT and nothing is overwritten. Of two agents creating the same new file, exactly one succeeds.
A refused write protects both sides: the agent re-reads the file, applies its change to the current content and writes again. The only path-level restriction is a Workstream hold, which a running phase can set on its paths for at most 14 days, and even then others can still write by giving a reason.