build: prepare crates.io release workflow

[muscariello]

Summary

• switch Rust publishing automation to a release-plz workflow patterned after SLIM
• make internal workspace dependencies publishable by adding explicit crate versions
• update repository metadata to agntcy/a2a-rs

Notes

• publication still depends on the GitHub secret CARGO_REGISTRY_TOKEN being a crates.io token for the artifacts-agntcy owner
• local dry-run packaging still depends on sibling crates not yet existing on crates.io, which is why this branch switches to ordered multi-crate release automation

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