docs: answer the git worktrees objection

Searching what people actually recommend today for "my agents keep clobbering
each other", the dominant answer is git worktrees. It is good advice and widely
adopted, which makes it the first thing anyone will say in response to this
project. Answering it up front, and conceding the true part first.

Worktrees isolate working directories on one machine, which is a real fix for one
developer running several agents. They do not reach across machines, because the
only state two clones share is the remote and git reports collisions at merge,
after both agents already did the work. They also isolate files rather than
intent, so two agents can implement the same feature in perfect isolation and you
pay twice.

They compose rather than compete.