Use #[tokio::main] instead of hand-rolled Runtime::new().block_on()

[demoray]

Both binaries previously stood up a tokio Runtime by hand, mapped
io::Error to Error::Io, and called .block_on(...). Replace with
#[tokio::main(flavor = "current_thread")], which is the idiomatic
shape and drops the manual error-mapping sites.

• src/bin/avml-upload.rs: collapse the separate async fn run into
  main. The binary already requires both put and blobstore, so
  no cfg work is needed.
• src/bin/avml.rs: tokio is feature-gated. Extract the synchronous
  snapshot pipeline into acquire(&Config), then provide two
  cfg-disjoint mains: a sync one when neither upload feature is
  enabled, and a #[tokio::main] async one when either is. The
  blocking snapshot work runs synchronously inside block_on before
  the first .await, matching the existing execution shape.
• Cargo.toml: add macros to tokio's optional feature set. Without
  it, --features put alone fails to find tokio::main. Other
  feature combinations were transitively pulling it in already.

Verified with:

• cargo build --no-default-features
• cargo build --no-default-features --features put
• cargo build --no-default-features --features blobstore
• cargo clippy --all-features --all-targets -- -D warnings
• cargo test --all-features
• cargo fmt --check