Require MCP authorization in rust_verify

[kiranandcode]

Summary

• reject direct rust_verify invocation unless --mcp is present
• consume the authorization flag before normal Verus argument parsing
• preserve authorized cargo-verus, wrapper, trait-check, vstd-build, and test paths
• update direct-invocation documentation

Tests

• cargo test -p rust_verify --bin rust_verify
• cargo test -p cargo-verus
• cargo test -p vstd_build
• cargo clippy -p rust_verify -p verus -p cargo-verus -p vstd_build --all-targets -- -D warnings
• cargo fmt --all -- --check
• manual rust_verify --version rejection and rust_verify --mcp --version acceptance

Assisted-by: Codex:GPT-5

[yiyunliu]

lgtm. will merge once the CI passes