feat: add macOS-26 to supported ImageOS mappings

[petermm]

Add macos26 to the ImageOSToContainer map so runners using macOS-26 are recognized instead of throwing an error.

Description

Allow macos-26

• [ x] I have read and understood the contributing guidelines

[petermm]

we ended up doing:

    - name: "Normalize ImageOS for setup-beam"
      if: matrix.os == 'macos-26'
      run: echo "ImageOS=macos15" >> "$GITHUB_ENV"

to get going.. https://github.com/atomvm/AtomVM/pull/2183/changes

[paulo-ferraz-oliveira]

Thanks. If tests pass I'll merge soon-ish. Ping me otherwise.

[paulo-ferraz-oliveira]

Tests fail, but it's most likely just a missing npm run build-dist. You might wanna rebase, too. Contributing guidelines (not always clear, I understand) mention "Execute npm run build-dist and fix any issues arising from that".

[petermm]

Thanks, hadn't noticed/understood..

Added macos-26 to CI, and updated readme.