feat: add custom error classes for better error handling

[alobroke]

Closes #9

What

Creates src/errors.ts with a custom error hierarchy for Passmark.

New classes:

• PassmarkError — base class, all errors extend this. Has a code property for programmatic identification
• StepExecutionError — step failed during AI or cached execution. Includes stepDescription field
• AIModelError — AI provider/model failure or unknown provider
• CacheError — Redis/cache failures (ready for future use as Redis handling grows)
• ConfigurationError — missing API keys or environment variables
• ValidationError — invalid inputs (missing script content, missing executionId/projectId)

Why

All errors were generic Error instances, making it impossible for users to programmatically distinguish between different failure types. Now users can do:

import { StepExecutionError, ConfigurationError } from "passmark";

try {
  await runSteps({ ... });
} catch (e) {
  if (e instanceof StepExecutionError) {
    console.log("Failed step:", e.stepDescription);
    console.log("Error code:", e.code); // "STEP_EXECUTION_FAILED"
  }
  if (e instanceof ConfigurationError) {
    // missing API key — show setup instructions
  }
}

Changes

• src/errors.ts — new file with all error classes
• src/index.ts — uses StepExecutionError and ValidationError
• src/models.ts — uses ConfigurationError and AIModelError
• src/data-cache.ts — uses ConfigurationError and ValidationError
• src/email.ts — uses ConfigurationError and AIModelError
• All error classes exported from the public API