[US-054-T1] Implement bundle validation engine with TDD

[KochTobi-Agent]

Task Description

Implement the core bundle validation engine for the oc bundle validate command using TDD (red-to-green approach).

TDD Cycle

RED Phase - Write Failing Tests

Create comprehensive tests in internal/bundle/bundle_test.go for ValidateBundle() function:

• TestValidateBundle_ValidBundle - valid bundle passes validation
• TestValidateBundle_MissingManifest - missing opencode-bundle.manifest.json returns error
• TestValidateBundle_InvalidJSON - malformed JSON in manifest returns error
• TestValidateBundle_InvalidManifestVersion - unsupported manifest_version returns error
• TestValidateBundle_MissingRequiredFields - missing bundle_name/bundle_version/presets detected
• TestValidateBundle_MissingEntrypoint - preset entrypoint file does not exist
• TestValidateBundle_InvalidPresetJSON - preset file exists but contains invalid JSON
• TestValidateBundle_MissingPromptFile - prompt_files entry references non-existent file

GREEN Phase - Implement to Pass

Add to internal/bundle/bundle.go:

• ValidationError struct (Category, Message, File fields)
• ValidationResult struct (Errors []ValidationError, Valid bool, Warnings []string)
• ValidateBundle(bundleRoot string) (*ValidationResult, error) function

Validation logic per REQ-F-022:

1. Check manifest file exists at opencode-bundle.manifest.json
2. Validate manifest is valid JSON
3. Validate against JSON schema (embedded 1.0.0.schema.json)
4. For each preset: verify entrypoint path exists
5. For each preset: verify all prompt_files paths exist
6. Validate each preset JSON file is valid JSON

Acceptance Criteria

• All tests written and initially failing
• Implementation passes all tests
• Exit code semantics: validation errors return non-nil result with Valid=false
• Error categories: manifest, schema, entrypoint, prompt_files, preset
• Follows existing code patterns in internal/bundle/