fix(init): correct RTK.md summary output

[ekkoitac]

Summary

• replace the circular RTK.md pointer to CLAUDE.md with a direct rtk --help command reference
• report the generated RTK.md line count from the embedded template instead of hard-coding 10 lines
• add regression coverage for the slim RTK.md reference and line-count helper

Fixes #2734

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets
• cargo test --all
• manual smoke: CLAUDE_CONFIG_DIR=$(mktemp -d) cargo run --quiet -- init -g --auto-patch prints RTK.md ... (29 lines); generated RTK.md is 29 lines and ends with Run \rtk --help` for the full command reference.`

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}