fix: handle advertised github commands

[nijanthanvijayakumar]

Summary

• handle the /cortex explain and /cortex test commands listed by /cortex help
• keep the existing placeholder style, but stop returning unknown-command responses
• add a small response helper so command output is unit-tested without GitHub calls

Bounty issue: PlatformNetwork/bounty-challenge#45894

Validation

• cargo fmt --package cortex-cli --check
• cargo test -p cortex-cli github_cmd::tests -- --nocapture
• cargo check -p cortex-cli --lib
• git diff --check
• Docker Desktop:

docker run --rm -v "$PWD":/work -w /work -e CARGO_TARGET_DIR=/tmp/cortex-target rust:1.95-bookworm bash -lc 'export PATH=/usr/local/cargo/bin:$PATH; apt-get update; apt-get install -y pkg-config libasound2-dev; rustup component add rustfmt; cargo fmt --package cortex-cli --check; cargo test -p cortex-cli github_cmd::tests -- --nocapture'

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 975 filtered out; finished in 0.01s

Notes

• existing unrelated warnings remain in cortex-tui/src/external_editor.rs and cortex-cli/src/utils/notification.rs