Feature: allow /compact to be invoked by the agent (programmatic compaction)

## Summary
`/compact` is currently a user-only slash command. The agent itself should be able to invoke compaction when it detects it's running short on context or when a long task naturally hits a phase boundary.

## Use cases
- Multi-phase workflows (e.g. plan → implement → review pipelines) where the agent knows when a phase is complete and the prior context can be safely summarized.
- Long-running autonomous sessions where the user isn't present to trigger `/compact` manually.
- Skill / plugin authors who want to script a compaction step at a known-safe boundary.

## Proposal
Expose `/compact` as a callable tool (e.g. `compact_context`) so the agent can invoke it programmatically, with the same effect as the user typing the slash command. Permission-gate it the same way other state-changing tools are gated.

## Out of scope
Auto-compaction policy (when to fire, what to summarize) is separate; this issue is only about exposing the existing capability to the agent.

Comienza localizando el comando /compact existente y las rutas de registro y control de permisos para las herramientas que cambian el estado. Verifica cómo la herramienta invocable puede producir el mismo efecto que el comando del usuario y, después, revisa las pruebas relevantes de CLI o los patrones de prueba. Se considera terminado cuando el agente puede invocar la compactación con el mismo control de permisos, sin añadir una política de compactación automática.

Escrito por el modelo de indexación a partir del texto del issue.