feat(runtime): stabilize configurable agent runtime interface #100

Goal

Create a stable, configuration-driven runtime boundary in Keen Code so alternative front ends can run the standard engine without forking the REPL, LLM, tool, MCP, session, and permission implementations.

Scope

• Define a narrow agent configuration/descriptor contract for the behavior currently supplied by Keen Agent:
  • identity and display (Name, optional ASCII art, agent slug)
  • system prompts and plan/build mode defaults
  • extra skills, subagent, and MCP configuration directories
  • excluded built-in tools
  • BTW and adversary configuration
• Define runtime options for interactive and headless execution.
• Refactor Keen Code's existing CLI path to consume this contract internally, using default behavior when no descriptor is supplied.
• Keep the initial API deliberately narrow; preserve dependency injection points for future tool registration or prompt-building overrides.

Acceptance criteria

• The keen CLI retains current behavior without an agent descriptor.
• Interactive and headless paths can receive an optional descriptor/options object.
• Agent-specific behavior is no longer coupled directly to REPL internals.
• Tests cover default behavior and at least one configured-descriptor path.

Follow-up

Once this is stable, expose it from an importable public package (tracked separately) for Keen Agent to consume.