chore: bump version to 1.3.1

Hardening release. No public API changes — same 5 NODE_CLASS_MAPPINGS
keys as v1.3.0. The bump exists so the registry can serve a build that
includes the version-tag guard, the registration contract tests, the
mapping drift self-check, the modernized typing, and the CI fixes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pyproject.toml

@@ -1,7 +1,7 @@
 [project]
 name = "comfyui-prompt-generator"
 description = "Generate Stable Diffusion prompts using Qwen3-8B via Ollama with 9 style presets"
-version = "1.3.0"
+version = "1.3.1"
 license = {file = "LICENSE"}
 readme = "README.md"
 requires-python = ">=3.10"