refactor: extract shared validation utilities and expand agent frontmatter support

[marcusrbrown]

• Create validation.ts module with shared type guards and utilities

  • Add PermissionConfig interface and related type guards (isRecord, isPermissionSetting, isToolsMap, isAgentMode)
  • Extract normalizePermission with helper functions to reduce complexity
  • Support AgentMode type ('subagent' | 'primary' | 'all')

• Expand AgentFrontmatter interface with 10+ optional fields

  • Add model, temperature, top_p, tools, disable, mode, color, maxSteps, permission
  • Implement extraction with type-safe helper functions (extractString, extractNumber, extractBoolean)
  • Add early return for parse errors

• Update agent conversion logic

  • Add addOptionalFields helper to reduce complexity
  • Support preserving explicit frontmatter mode over CLI options
  • Extract all optional fields to OpenCode config in config-handler

• Replace stripFrontmatter with parseFrontmatter().body.trim()

  • Update bootstrap.ts, config-handler.ts, skill-loader.ts
  • Add JSDoc to stripFrontmatter clarifying it's a convenience wrapper

• Refactor CLI to import AgentMode from validation.ts

• Add comprehensive test coverage

  • 24 tests for agent frontmatter extraction (agents.test.ts)
  • 6 new tests for config handler field extraction
  • 10 new tests for converter frontmatter transformation
  • Tests for edge cases: mode: all, invalid values, empty permissions

All 151 tests passing. Complexity reduced below Biome limit (15).