ci: remove unecessary cleaning step and make e2e-smoke and swift-integration only run when needed

[Lyokone]

• Drop the dead disk-cleanup steps. Free Disk Space (Ubuntu) reclaimed 18 GB on a runner that already had 81 GB free, costing 72s on every android job (×4 workflows); the iOS Free up space step rm -rf'd Linux paths on a macOS runner — a 0.46s no-op — so its ios-free-up-space input and firestore's opt-in go too.
• Scope the smoke suite and all_plugins by platform. New reusable_repo_changes.yaml (repo-wide sibling of reusable_e2e_changes.yaml, which is per-package) maps changed paths to platforms, so a Kotlin-only change runs the android jobs and skips ios/macos/web/windows — including the two 35-min swift-integration-* macOS jobs. analyze/format/test stay ungated as pure Dart.

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