goal: add Copy/View full to the pill menu and keep the objective readable after Clear #61333

Description

The goal pill is the only place that shows the full goal objective, but its popover heading is clamped to three lines and the menu has no Copy or "View full" action. Clearing the goal deletes the goal state, hides the pill, and leaves the truncated timeline notice as the only record.

Steps to reproduce

1. Run /goal <long objective>.
2. Click the Goal / Goal met pill above the composer.
3. Observe the three-line heading; try to copy the full objective or find a copy action.
4. Click Clear.
5. The pill disappears. The timeline still shows only Goal: <truncated>.

Expected

• A Copy action copies the complete objective.
• A "View full" action or scrollable view shows the complete objective.
• Clearing the goal does not remove the only readable copy; the transcript should stay self-sufficient.
• Clear should ask for confirmation, since it is destructive.

Actual

• The heading uses line-clamp-3 at 11px in a 320px popover.
• The menu only offers Pause, Resume, and Clear.
• Clear removes the goal state; the full objective remains in messages[0].text but has no UI entry point.

Root cause

• packages/app/src/session/composer/plugin-pills.tsx:110-119: generic popover header with w-80, text-[11px], and line-clamp-3.
• packages/gui-plugins/src/goal/renderer.tsx:94-112: menu only has Pause, Resume, and Clear.
• packages/core/src/plugin/goal.ts:374-383: clear writes undefined to the goal storage.

Additional context

Built-in goal plugin (#59437). The same objective is also hidden in the timeline; see #61332.

Environment

• OpenCode Desktop v2.2.5 (source checkout 1bf08ae; Desktop and web share the same packages/app renderer).
• Windows desktop app.
• No third-party plugins; this is the built-in goal plugin.