Forge v1.9.0 — Field format semantics

Added

• MCPField.Format string — populated from the forge_format struct tag; machine-readable
  format hint ("markdown", "html"). Empty string when the tag is absent (Decision 27).
• MCPField.Description string — populated from the forge_description struct tag; free-text
  authoring guidance for AI agents. Empty string when the tag is absent (Decision 27).
• forge-mcp: tool input schemas now emit a "description" key in JSON Schema properties
  using the priority logic defined in Decision 27:
  both tags → forge_description + " (" + forge_format + ")";
  format only → "(" + forge_format + ")"; neither → key omitted.