fix(ci): use static DMG filename without version suffix

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What

The release DMG was named with the version appended (e.g. ForgeCode-0.1.3.dmg). This changes it to a simple, static ForgeCode.dmg.

Changes

• macos/scripts/common.sh — DMG_NAME default is now ForgeCode instead of ForgeCode-$APP_VERSION. This single variable drives DMG_PATH, staging temp file names, and the manifest dmg_name field, so the rest of the packaging scripts follow automatically.
• .github/workflows/release.yml
  • The "Derive APP_VERSION" step now emits DMG_NAME=ForgeCode$suffix / dmg_file=ForgeCode$suffix.dmg. The -unsigned suffix for PR dry runs is preserved.
  • The draft-release asset cleanup jq filter matches startswith("ForgeCode") rather than startswith("ForgeCode-"), so previously-uploaded versioned assets are still matched and pruned.

Notes

• Sparkle is unaffected. The appcast <enclosure url> is tag-scoped (releases/download/$TAG_NAME/$DMG_FILE), and the update version comes from sparkle:shortVersionString / sparkle:version, not from the filename. A fixed filename per tag is safe.
• The version remains discoverable via the release tag, Info.plist, and manifest.txt.

Verification

sh scripts/test-packaging.sh passes all 18 checks, including "legacy fixed-name DMG staging residue is removed".