feat(prompt) support configurable prompt template directories

[zhoujh01]

Description

Related Issue

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

第一，给 prompt 模板目录加了可配置能力。PromptManager 现在会按这个优先级选目录：显式传入参数、环境变量
OPENVIKING_PROMPT_TEMPLATES_DIR、ov.conf 里的 prompts.templates_dir、最后才是内置目录。实现入口在 openviking/prompts/
manager.py:63，配置模型加在 openviking_cli/utils/config/open_viking_config.py:154 和 openviking_cli/utils/config/
prompts_config.py:8。

第二，把自定义模板目录做成了“局部覆盖”而不是“整套替换”。也就是用户只放改过的 prompt 文件就行，某个 prompt 在自定义目录里
找不到时，会自动回退到内置 openviking/prompts/templates。这个 fallback 逻辑在openviking/prompts/manager.py:141。

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

Additional Notes

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