feat(cli): add version and update checks

[owainlewis]

Summary

• add cortex --version and -V using the Cargo package version
• add explicit cortex --check-update against the latest GitHub Release
• treat no published releases as a successful no-update result and add curl timeouts for network failures

Verification

• /opt/homebrew/bin/cargo-fmt -- --check
• /opt/homebrew/bin/cargo-clippy clippy -- -D warnings
• cargo test
• cargo run -- --version
• cargo run -- --check-update with no releases published
• temporary release v9.9.9-version-test.20260622 verified update-available output, then release/tag deleted

Review

• Subagent review found 404/no-release and timeout gaps
• Follow-up subagent review found no issues after fixes

Closes #36