Replace Windows EXE installer with portable Book Importer bundle

[arieldaniely]

Motivation

• Replace the previous Windows-only installer workflow with a portable, no-installer distribution to simplify packaging and cross-machine usage.
• Preserve compatibility for existing automation and documentation that referenced the old EXE tasks and artifact names.

Description

• Updated GitHub Actions workflow to build and upload a portable bundle instead of a Windows EXE, renamed the job and artifact names, and changed the invoked Gradle task to packageBookImporterPortable.
• Added packageBookImporterPortable task to the root build.gradle.kts and made the old packageBookImporterWindowsExe a compatibility alias that depends on the portable task.
• In generator/bookimporter removed the native installer targetFormats, added a packagePortableBundle Zip task and packagePortable task that create a portable app folder and ZIP, and made packageWindowsExe a compatibility alias to packagePortable.
• Updated generator/bookimporter/README.md to document the portable bundle workflow and to note the legacy command compatibility for packageBookImporterWindowsExe and :bookimporter:packageWindowsExe.

Testing

• Updated CI workflow to trigger the new portable packaging job in .github/workflows/build-bookimporter-exe.yml and the workflow file passed YAML linting in the rollout context.
• Locally exercised packaging commands by running ./gradlew :bookimporter:packagePortable and ./gradlew packageBookImporterPortable which produced the portable app folder and ZIP in generator/bookimporter/build/compose/binaries/main-release/portable/ (success).

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Codex Task