Add lightweight /goal mode for session objectives

## Summary
Add a lightweight `/goal` slash command for session-scoped standing objectives. Unlike `/mission`, this should keep execution on the normal main-agent path and preserve existing tool approval safeguards.

## Proposed MVP
- `/goal ` stores one active goal for the current channel/user/session.
- `/goal status`, `/goal pause`, `/goal resume`, and `/goal clear` manage that state.
- Ordinary follow-up messages receive a small goal reminder while the goal is active.
- The main agent can mark the goal achieved with an explicit completion marker.

## Non-goals
- No Mission worker/verifier pipeline.
- No PRD/story loop.
- No bypass of normal tool guards or approvals.
- No separate judge model requirement in the MVP.

## Rationale
`/mission` is useful for structured autonomous engineering loops, but it is heavier than a persistent objective reminder. A constrained `/goal` gives users a durable session objective without changing the execution and security model.