Refactor commit status logic and strengthen SFDX commit guidance

[aruntyagiTutu]

• Extracted commit status logic to src/getCommitStatus.ts (fetchCommitStatus).
• Refactored src/tools/checkCommitStatus.ts to use the shared helper (no behavior change).
• Updated src/tools/sfDevopsCommitWorkItem.ts description to explicitly state: never use git commit for SFDX; always use commit_devops_center_work_item.

Rationale

• Aligns with existing pattern (getProjects.ts, getWorkItems.ts) for reusability and testability.
• Clarifies correct workflow so DevOps Center properly tracks metadata and ties commits to Work Items.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @aruntyagiTutu to sign the Salesforce Inc. Contributor License Agreement.