[feat] Add agent config file support for name, description, and custom prompt

[kyegomez]

Summary

Add support for a user-defined config file that allows customizing individual agents in the swarm.

Motivation

Currently, agent properties such as name, description, and system prompt are hardcoded. A config file would let users tailor the swarm to their specific use case without modifying source code.

Proposed Config Fields

agents:
  - name: "ResearchAgent"
    description: "Specializes in web research and summarization"
    system_prompt: "You are a research assistant. Your goal is to..."
    model: "claude-sonnet-4-6"
    max_tokens: 8096
    temperature: 0.7

  - name: "CoderAgent"
    description: "Writes and reviews code"
    system_prompt: "You are an expert software engineer..."
    model: "claude-opus-4-6"
    max_tokens: 16000

Acceptance Criteria

• Define config file format (YAML or TOML)
• Support fields: name, description, system_prompt, model, max_tokens, temperature
• Load config from a default path (e.g. claw_swarm.yaml or ~/.claw_swarm/config.yaml)
• Allow overriding config path via CLI flag (--config) or environment variable
• Validate config on load and surface helpful errors for missing/invalid fields
• Fall back to sensible defaults if optional fields are omitted
• Update CLI and API server to respect agent configs at startup
• Add documentation and an example config file

Notes

• Config should be optional — existing behavior should remain unchanged when no config is present
• Consider supporting multiple named agent profiles that can be selected at runtime