Fix CLI build contract issues

[sionsmith]

Summary

• make semrush the default Cargo run target
• allow dry-run and batch estimate without requiring an API key
• accept README-style underscore batch command names
• align documented exit codes and README command examples with the implemented CLI
• add CLI regression tests for the fixed behavior

Verification

• cargo fmt --all -- --check
• cargo check --all-targets
• cargo test --all-targets
• cargo clippy --all-targets -- -D warnings
• cargo build --release
• RUSTFLAGS='-D warnings' cargo test --all-targets\n\n## Notes\n- Live Semrush endpoint correctness is still not covered by mock HTTP contract tests; this PR focuses on build, CLI, and documentation-contract fixes.