Add Sparkle release pipeline + Signing/Notorization Actions

[FuJacob]

Summary

• Add a tag-triggered GitHub Actions release workflow for building, notarizing, Sparkle-signing, and publishing Tabby releases.
• Teach scripts/generate_appcast.py to sign with an explicit Sparkle Ed25519 key file for CI.
• Update RELEASING.md and the Sparkle feed URL for the custom-domain Pages appcast.

Validation

• python3 -m py_compile scripts/generate_appcast.py
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/release.yml"); puts "release.yml parsed"'
• plutil -lint TabbyInfo.plist
• git diff --check

Notes

• The workflow publishes only on v* tag pushes; manual dispatch defaults to validation without publishing.
• Closing CI: release pipeline (notarize + Ed25519 sign + appcast) #36 still requires a successful tag release run.