## Why
Users have requested the ability to edit a goal's objective after a goal
has been created. This PR exposes a new `/goal edit` command in the TUI
to address this request.
In the process of implementing this, I also noticed an existing bug in
the goal runtime. When a goal's objective is updated through the
`thread/goal/set` app server API, the goal runtime didn't emit a new
steering prompt to tell the agent about the new objective. This PR also
fixes this hole.
## What Changed
- Adds `/goal edit` in the TUI, opening an edit box prefilled with the
current goal objective.
- Keeps active and paused goals in their current state, resets completed
goals to active, keeps budget-limited goals budget-limited, and
preserves the existing token budget.
- Changes the existing `thread/goal/set` behavior so editing an
objective preserves goal accounting instead of resetting it. The older
reset-on-new-objective behavior was left over from before
`thread/goal/clear`; clients that need to reset accounting can now clear
the existing goal and create a new one.
- Reuses the existing goal set API path; this does not add or change
app-server protocol surface area.
- Adds a dedicated goal runtime steering prompt when an externally
persisted goal mutation changes the objective, so active turns receive
the updated objective.
## Validation
- Make sure `/goal edit` returns an error if no goal currently exists
- Make sure `/goal edit` displays an edit box that can be optionally
canceled with no side effects
- Make sure that an edited goal results in a steer so the agent starts
pursuing the new objective
- Make sure the new objective is reflected in the goal if you use
`/goal` to display the goal summary
- Make sure that `/goal edit` doesn't reset the token budget, time/token
accounting on the updated goal
Eric Traut