seed default SOUL and IDENTITY files for new agents

[jamiepine]

Summary

• seed new-agent SOUL.md from a Jinja template (prompts/en/identity/default_soul.md.j2) instead of a placeholder comment
• add a short starter IDENTITY.md Jinja template (prompts/en/identity/default_identity.md.j2) and render both templates during identity scaffolding
• update docs to describe the new defaults for SOUL.md, IDENTITY.md, and USER.md

Code Example

const DEFAULT_SOUL_TEMPLATE: &str = include_str!("../../prompts/en/identity/default_soul.md.j2");
const DEFAULT_IDENTITY_TEMPLATE: &str =
    include_str!("../../prompts/en/identity/default_identity.md.j2");

let rendered_soul = render_jinja_template("default_soul", DEFAULT_SOUL_TEMPLATE)?;
let rendered_identity = render_jinja_template("default_identity", DEFAULT_IDENTITY_TEMPLATE)?;

[jamiepine]

need to revise the default SOUL to not be discord oriented