Devbox sparse folder UX and cache intents

[Snowy7]

Summary

• Add product CLI sparse-folder commands for warm, hydrate, keep, free-space, and sparse clone.
• Surface folder status in user-facing hydrated, cloud-only, changed, kept, cleanable, and pending-upload terms.
• Reuse Loom cache/hydration primitives with dirty-change and remote-proof safety, plus docs for the current CLI sparse-folder model.

Tests

• cargo fmt --all
• cargo fmt --all -- --check
• cargo test -p devbox-cli --test product_cli
• cargo test -p loom-cli --test cli
• cargo test --workspace --quiet

[Snowy7]

Review result: APPROVED, but GitHub would not let this account submit an approving review because it owns the pull request.\n\nReviewed commit 5f772a3. No blocking findings. Verified the sparse product commands keep normal output in folder/product terms, keep/pin does not claim hydration, free-space requires remote proof and skips dirty/kept/unsupported local entries, remote-only files are preserved as non-deletions, and secret/generated/dependency policies remain intact.\n\nTests run: cargo fmt --all -- --check; cargo test -p devbox-cli --test product_cli; cargo test -p loom-cli --test cli; cargo test --workspace --quiet.\n\nResidual risk: PR is still marked draft, but the reviewed code path is safe to merge once the draft state is resolved.