Publish to PyPI as da-sync, not da-cli

[FZ2000]

PyPI refuses da-cli: it is too similar to the unrelated dacli
project, which the two differing only by a separator is precisely what
its typosquat guard exists to catch.

Worth recording, because it invalidates the check I had been relying on:
https://pypi.org/pypi/da-cli/json returns 404. The name is not
taken. It is still rejected. Availability and acceptability are separate
questions on PyPI, and only the pending-publisher form answers the
second — so tools/check_discoverability.py-style 404 probes cannot
clear a name on their own.

da-sync instead: seven characters, says what the tool does, reads
naturally with the da command, and differs from dacli by more than a
separator, which is the specific thing that tripped the guard.

THREE NAMES, THREE JOBS — this is the part worth being explicit about,
because they are now deliberately different and that looks like a mistake
if you do not know why:

distribution (PyPI) da-sync what you pip install
import package dacli what import resolves; unchanged
console script da what you type; unchanged
repository da-cli unchanged

Only the first changed. Nothing about the installed tool moves.

Verified by building and installing the wheel into a clean virtualenv
rather than reasoning about it:

built da_sync-0.1.0-py3-none-any.whl, da_sync-0.1.0.tar.gz
da --version -> da-cli 0.1.0
import dacli -> ok, version 0.1.0
metadata -> name=da-sync version=0.1.0

The existing dacli on PyPI installs a top-level dacli/ directory and
a dacli console script. So our console script does not collide, but the
import package would if both were installed into the same environment —
pip does not detect two distributions writing the same directory. That is
accepted deliberately: this is an application, not a library, nobody
imports it, the README recommends pipx (which isolates per tool), and the
two audiences barely overlap.

The README's install section now names da-sync and warns about both
neighbouring names rather than only one.

tools/check_version_sync.py matches both da-cli and da-sync when it
scans prose for a version that disagrees with the package — the first is
still the HTTP User-Agent and the repository name, the second is now the
distribution.