chore: migrate rust/inter-canister-calls to icp-cli

[marc0olo]

Summary

• Replaces dfx.json with icp.yaml using @dfinity/rust@v3.3.0 for both counter and caller canisters
• Moves source from src/caller/ and src/counter/ into top-level caller/ and counter/ subdirs
• Updates workspace Cargo.toml, per-crate Cargo.toml files (ic-cdk 0.20, removes ic-cdk-macros), and regenerates Cargo.lock
• Adds rust-toolchain.toml with wasm32-unknown-unknown target
• Replaces dfx-based Makefile with icp canister call tests (10 tests covering all public methods)
• Adds new CI workflow inter-canister-calls.yml with ghcr.io/dfinity/icp-dev-env-rust:1.0.0; deletes legacy rust-inter-canister-calls.yaml
• Deletes dfx.json, BUILD.md, .devcontainer/, src/caller/caller.did, src/counter/counter.did
• Updates README to use icp-cli commands

Test plan

• CI passes
• icp network start -d && icp deploy && make test succeeds locally

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