Generate Flatpak manifest

[Photon101]

Summary

• Replace the Flatpak target placeholder build with generated flatpak-builder manifest output
• Support runtime/sdk metadata, branch selection, SDK extensions, finish args, build commands, and archive or local directory sources
• Add focused tests for generated manifest content and dry-run shipping

Validation

• pnpm vitest run packages/targets/pkg-flatpak/src/index.test.ts
• pnpm --filter @profullstack/sh1pt-target-pkg-flatpak typecheck
• pnpm --filter @profullstack/sh1pt-target-pkg-flatpak build
• git diff --check