Add support for supplying custom system prompt to the underlying tooling

[ridafkih]

Currently, there's no way to support supplying a system prompt

• Claude supports this through the --append-system-prompt argument
• Codex supports this through the config.toml model_instructions_file
• OpenCode supports this through the prompt config.
• Amp does not support this.
• Cursor Agent does not support this.

[linear]

RVT-5907

[ridafkih]

@NathanFlurry I can take this if you're alright with that