System Prompt Customization

Agents are defined inside ~/.hiroshi/AGENTS.md and load their instructions dynamically on session creation.

## Developer
- Prompt: "You are a systems programmer. Write zero-warning Rust code."
- Allowed Tools: [cargo_check, git_manager]

Hiroshi parses the markdown file on startup:

System Prompts: The prompt string is loaded into the model’s system context.
Allowed Tools validation: Pre-compiled list of allowed commands prevents authorization leaks.
Handoff parameters: Rules specifying when to yield control to other agents.

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