Update developer and PRD documentation to reflect the new structured markdown
error formatting with explanations and examples.

## Changes

### Developer Guide (docs/errors.md)

**Error Builder API**:
- Document `WithExplanation()` and `WithExplanationf()` methods
- Document `WithExample()` and `WithExampleFile()` methods
- Add embedded markdown example pattern with `//go:embed`
- Update Quick Start with complete example using all features

**Error Formatting**:
- Add "Structured Markdown Output" section showing 6 sections
- Document section order and conditional rendering
- Provide complete example with all sections
- Update configuration options to reflect context table in all modes

### PRD (docs/prd/atmos-error-handling.md)

**Design Decisions**:
- Update "Builder Pattern for Complex Errors" with new methods
- Add "Structured Markdown Error Presentation" as Decision #5
- Document section hierarchy and visual design rationale
- Provide formatted example output

**Error Categories**:
- Update "Builder-Enhanced Errors" with rich error example
- Show explanations, examples, hints, and context together
- Document use cases for structured error presentation

**Migration Path**:
- Add Phase 6: Structured Markdown Error Formatting (Complete)
- Document all implementation details:
  - New builder methods
  - Formatter refactoring with 6 sections
  - Workflow error conversions
  - Exit code handling updates
  - 21 comprehensive tests
  - Golden snapshot regeneration

All documentation now reflects the complete structured markdown error system.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>