fix(core): enhance shell agent-file exception robustness

[StoneChin]

Related Issue

Resolve #826

Description

#826

1. Following Agent 配置包含 subagents 时启动静默失败（Exit code 1，无错误输出） || Agent configuration fails to start silently when it contains subagents (Exit code 1, no error output) #826 thought, I discovered more related bugs. Incorrect document formatting may cause bugs like empty yaml file or the file contains only one line of string. This bug is caused by incorrect return type of yaml.safe_load() in file src/kimi-cli/agentspec.py
2. Referring to the agent usage logic of gemini-cli, when a sub-agent error occurs, an error is output in the terminal, and the system starts with default parameters. Therefore, I believe that the logic for using agent-file should be changed to something similar.
3. add src/kimi-cli/exception.py simple exception log example for Agent 配置包含 subagents 时启动静默失败（Exit code 1，无错误输出） || Agent configuration fails to start silently when it contains subagents (Exit code 1, no error output) #826. (Maybe it's not necessary.)

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog.
• I have run make gen-docs to update the user documentation.

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[devin-ai-integration]

Devin Review found 2 potential issues.

View issues and 3 additional flags in Devin Review.