ci: emit SHA256SUMS.txt alongside per-asset sidecars

[scott]

Motivation

Every release currently ships one .sha256 sidecar per binary. That's enough for install.sh and gander --upgrade, but Homebrew formulas and most distro packagers expect a single combined SHA256SUMS.txt file with <hash> <asset> lines. Adding this is a small step that unlocks packaging work without changing any consumer code.

Acceptance criteria

1. New release step in .github/workflows/release.yml (in the release job, before Create release) that:
   • Concatenates every per-asset checksum plus the new windows-* assets (after docs: add a man page (gander.1) #30 lands) into a single SHA256SUMS.txt.
   • Sorts entries deterministically (e.g. ls release-assets/gander-* | sort | xargs sha256sum).
2. SHA256SUMS.txt is uploaded as a release artifact alongside the per-asset sidecars.
3. Format is the GNU coreutils sha256sum default: two-space separator between hash and filename.
4. release.yml's files: list under softprops/action-gh-release@v3 includes release-assets/SHA256SUMS.txt.
5. Existing install.sh and gander --upgrade continue to use the per-asset sidecars — no behavior change for end-users.

Out of scope

• Signature verification. That's ci: sign release artifacts with cosign (keyless, OIDC) #33 / feat: verify cosign signature in gander --upgrade #34.