[plan] Convert single-value enums to const and add format constraints

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Objective

Improve schema quality by converting single-value enum arrays to const (JSON Schema best practice) and adding format constraints where appropriate.

Context

This addresses Issues #5 and #6 (Low priority) from the schema audit:

• 10 instances use "enum": ["value"] instead of "const": "value"
• Zero format constraints despite opportunities for stronger validation (email, uri, date-time, etc.)

These are minor refinements but improve schema correctness and validation strength.

Approach

Part 1: Convert Single-Value Enums

Find all instances of "enum": ["single-value"] and replace with "const": "single-value".

Expected pattern:

// Before
"type": {
  "enum": ["stdio"]
}

// After
"type": {
  "const": "stdio"
}

Part 2: Add Format Constraints

Identify fields that would benefit from format constraints:

• URLs → "format": "uri"
• Email addresses → "format": "email"
• Date/time fields → "format": "date-time"
• Relative references → "format": "uri-reference"

Note: Only add format constraints where validation makes sense. Don't add them just to have them.

Files to Modify

• pkg/parser/schemas/main_workflow_schema.json
• pkg/parser/schemas/included_file_schema.json
• pkg/parser/schemas/mcp_config_schema.json

Implementation Steps

1. Search for "enum": [ with single-element arrays
2. Replace with "const": (10 expected instances)
3. Review string fields for format constraint opportunities
4. Add format constraints to URL, email, and date-time fields
5. Test that validation behavior remains correct

Acceptance Criteria

• All single-value enums converted to const
• Format constraints added to appropriate string fields (URLs, dates, etc.)
• Schema validation still passes
• No breaking changes to validation behavior
• Run make build and make test to verify everything compiles and tests pass

Estimated Effort

1-2 hours (includes careful validation testing)

AI generated by Plan Command for discussion #8130