feat: add xtask crate with publish order command

[mircea-c]

Replaces the Python script with a Rust-based cargo xtask command backed by the
anza-xyz/xtask library, keeping the implementation in one place.

Changes

• Add ci/xtask crate depending on anza-xyz/xtask for publish order logic
• Replace ci/order-crates-for-publishing.py with cargo xtask publish order
• Update .github/workflows/lint.yml to use the new command
• Exclude ci/xtask from cargo-for-all-lock-files.sh to avoid passing agave-specific feature flags to an unrelated workspace