feat: implement error handler

[shepherd-001]

Issue

Closes #25

Root cause

What changed and why

Definition of done — addressed item by item

• Item 1: ...
• Item 2: ...
• Item 3: ...

Evidence this actually runs

paste test/build output here

Tests

Regression check

Checklist

• [ x ] Every Definition of done bullet above is checked and explained, not just checked
• [ x ] Evidence block above is filled in with real output, not omitted
• [ x ] New/updated tests are included and shown passing
• [ x ] No leftover console.log/TODO/debug code
• [ x ] Related/adjacent behavior re-verified, not assumed unaffected