docs: README and setup guide still read as if 0.1.0 is the latest release #117

The README and docs/setup.md were written for 0.1.0. 0.2.0 came out on 2026-10-17, and the next release is waiting in #113. Some places still read as if 0.1.0 were the latest release:

• README, Status: "the first release, 0.1.0, is out". It names no later release.
• README, Quick start step 5, and docs/setup.md §8 (lines 933–938): the example takes the systemd unit from refs/tags/v0.1.0 ("For the release 0.1.0: …").
• docs/setup.md lines 193–204: the by-hand example uses the file nodkey_0.1.0_linux_amd64.tar.gz.
• docs/setup.md line 1152: the example log line shows version=0.1.0.

The harm is small. The 0.1.0 and 0.2.0 unit files differ only in a comment. But a new reader may think 0.1.0 is the latest release and install it by hand.

These notes are still true and can stay: 0.1.0 has no Sigstore bundle (README step 1, docs/setup.md line 176), and 0.1.0 has no nodkey update (docs/setup.md line 1466). docs/releasing.md names 0.1.0 as history; that is fine too.

Related: #81 (closed).

Done when

• The README status names no release number, or a number that is updated at each release, so it does not go out of date again.
• The unit examples (README step 5, docs/setup.md §8) tell the reader to take the unit from the tag of the version they installed, with an example that does not go out of date.
• grep -n '0\.1\.0' README.md docs/setup.md finds only the intended notes about 0.1.0.