fix: match github cortex mentions case-insensitively

[nijanthanvijayakumar]

Summary

• make GitHub comment trigger matching case-insensitive for @Cortex and /Cortex
• normalize parsed GitHub commands so @Cortex Fix maps to fix
• fix the existing agent test helper import so focused CLI tests compile

Bounty issue: PlatformNetwork/bounty-challenge#44439

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 "$PWD":/work -w /work -e CARGO_TARGET_DIR=/tmp/cortex-target rust:1.95-bookworm bash -c 'export PATH=/usr/local/cargo/bin:$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'

Screenshots / proof

Docker result:

running 6 tests
test github_cmd::tests::test_get_help_message ... ok
test github_cmd::tests::test_parse_cortex_command ... ok
test github_cmd::tests::test_contains_cortex_trigger_case_insensitive ... ok
test github_cmd::tests::test_install_validates_path_is_directory ... ok
test github_cmd::tests::test_install_accepts_valid_directory ... ok
test github_cmd::tests::test_install_validates_path_exists ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 975 filtered out

Notes

• Contributing guide checked: docs/CONTRIBUTING.md
• PR title uses the repo's fix: convention
• Branch has two focused commits