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.

Start by locating the existing /compact command and the registration and permission-gating paths for state-changing tools. Verify how the callable tool can produce the same effect as the user command, then check the relevant CLI tests or test patterns. Done means the agent can invoke compaction with the same permission gate, without adding automatic compaction policy.

Written by the indexing model from the issue text.