Feature Request: Allow project-level custom system prompt

What feature would you like to see?

Allow users to define a custom \system_prompt.md\ at the project level. If the file exists and is valid, it would override the default system prompt.

Suggested behavior:

• Scan for \system_prompt.md\ in the following locations (from project root to current working directory, leaf-most wins):
  • .kimi/system_prompt.md\
  • .agent/system_prompt.md\
  • .claude/system_prompt.md\
• If found, load it through the existing Jinja2 template pipeline so built-in variables like \\ and \\ still work.
• Apply basic validation: non-empty, size limit (e.g. 64 KiB), valid template syntax.
• Existing sessions should continue using their persisted system prompt (from context) to avoid disruption.

Additional information

Kimi models are becoming increasingly capable and can handle a wide range of tasks beyond coding—such as writing lesson plans, financial reports, and creative content. However, the current built-in system prompt is strongly coding-oriented, which limits the CLI's usefulness for these other scenarios.

This would let users tailor the agent's personality and instructions to their specific domain without needing to maintain a full custom agent spec YAML.

Start by tracing the existing system prompt loading and Jinja2 template pipeline, including how persisted prompts are read from context. Then inspect the project-root locations for .kimi/system_prompt.md, .agent/system_prompt.md, and .claude/system_prompt.md; done means the leaf-most valid file overrides the default while existing sessions retain their persisted prompt.