Fix goal update and add /goal edit command in TUI

[etraut-openai]

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

[canvrno-oai]

When testing /goal edit, it did reset the Time Used/Tokens Used values for the edited goal. The edits were successfully applied and steered, and edits were cancellable.

[etraut-openai]

@canvrno-oai, good catch! I forgot that my original implementation cleared the goal time/token accounting when the objective was updated. That design decision no longer makes sense for the thread/goal/set api because I have since added a dedicated thread/goal/clear api that clients can use if they want to start a new goal (including resetting its account stats). I've updated the code in the PR accordingly.