v5.2.1: Require the signed packslip for mise 2026.9.3 and later

The action now refuses to install mise 2026.9.3 or later if the release's signed packslip is missing. This closes a gap where verification could be downgraded to the weaker checksum check.

Fixed

• A missing packslip no longer weakens the check on recent mise releases. v5.2.0 started checking the mise binary against the release's signed packslip.sigstore.json. If that file wasn't found, though, the action quietly fell back to the minisign-signed SHASUMS256 checksums. Anyone who could hide the file could force the weaker check. Every mise release from 2026.9.3 on includes a packslip, so for those versions a missing bundle now stops the install with an error like this:

  mise v2026.10.11 must ship a signed packslip, but https://github.com/jdx/mise/releases/download/v2026.10.11/packslip.sigstore.json was not found
  

  Releases before 2026.9.3 don't include a packslip, so they're still checked against the signed checksums. A version string that isn't three dot-separated numbers also requires a packslip. If you set the sha256 input, it still takes precedence over both checks. A packslip that fails verification still never falls back to the checksums. (#695 by @jdx)

Full Changelog: v5.2.0...v5.2.1