feat: implement non-UI execution contract for deal-lead-mail-tracker

[dev-king-001]

PR Title:
feat: implement non-UI execution contract for deal-lead-mail-tracker

PR Description:

What was done

• Defined a stable, typed execution contract (DealLeadTrackerInput and DealLeadTrackerOutput) along with standard error codes and boundaries.
• Exported a non-UI service entry point executeDealLeadMailTracker in service.ts and wired it up via index.ts.
• Implemented fixtures (successFixture and failureFixtureMissingInput) that cover basic success and validation failure cases.

Why it was done

• The deal-lead-mail-tracker tool needed a stable backend-facing contract to run independently of presentation concerns, ensuring reliable execution gaps are covered before further UI integration.

How it was verified

• Verified that all exports provide the correct typings and validation logic executes.
• Ensured absolutely no visual, layout, or design system components were changed, respecting the scope.

Closes #1364