feat: add agent first-prompt creation flag

[HonestMajority]

What

• Renames project-scoped agent creation prompts to --first-prompt across the generic and role-specific CLI, with the old --description flag retained as a hidden compatibility alias.
• Lets omitted or blank first prompts create idle project-scoped agents with no initial inbox message, while nonblank prompts still seed exactly one initial message.
• Updates the TUI agent wizard, README examples, and embedded CoS/PM guidance to describe first prompts and idle agents.

Why

--description sounded like metadata, but the value is used as the agent's first inbox prompt. This aligns project-scoped agent creation with task creation without changing project creation, task creation, or the agent storage schema.