fix: detect Hermes agent mode and restore child SIGINT

[seungwonme]

Summary

• Auto-detect Hermes Agent child commands with HERMES_SESSION_ID / HERMES_SESSION_KEY so get hides secret values without extra opt-in.
• Restore the wrapped child process's SIGINT disposition to default before exec, while the wrapper ignores Ctrl-C and waits.
• Add regression coverage for Hermes markers and child SIGINT handling.
• Document Hermes support in Korean/English READMEs and the bundled agent skill.

Verification

• docker run --rm -v /Users/aiden/dev/agents-env:/repo -w /repo rust:1.88-bookworm bash -c 'export PATH=/usr/local/cargo/bin:$PATH; rustup component add clippy >/tmp/rustup-clippy.log; cargo test && cargo clippy --all-targets -- -D warnings'
• git diff --check

Note

cargo fmt --check on Rust 1.88 reports formatting diffs in pre-existing untouched files, so this PR keeps the diff focused instead of reformatting the repository.