Custom providers via config

[ComeBertrand]

Summary

Allow users to define agent providers in config.toml without writing Rust, so any CLI-based agent can be used with zinc.

Motivation

Currently adding a new agent requires implementing the Provider trait in zinc-daemon and releasing a new version. A config-based approach lets users add any agent immediately.

Proposed config format

[agents.my-agent]
command = "my-agent-cli"
idle_timeout = 5                  # seconds before "no output" = "idle"

Considerations

• Config-defined providers use the PTY idle heuristic for state detection (no hooks)
• The command field is the binary name; args, resume, and prompt mapping could be added later
• Should be validated and added to the known providers list so zinc spawn --agent my-agent works
• Hook support for custom providers is out of scope for the initial implementation