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.

Comece localizando o comando /compact existente e os caminhos de registro e controle de permissões para ferramentas que alteram o estado. Verifique como a ferramenta invocável pode produzir o mesmo efeito que o comando do usuário e, em seguida, confira os testes relevantes de CLI ou os padrões de teste. O trabalho estará concluído quando o agente puder invocar a compactação com o mesmo controle de permissões, sem adicionar uma política de compactação automática.

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