Agents: show the copied prompt in Describe, and let the trail jump to any step

[epeicher]

What it does

Two fixes to the agent creation wizard in WP Explorer, plus a hover state for <os-step>.

The system prompt was invisible for copied and drafted agents. "Start from someone" copied the source's instructions into the hidden instructions field and landed on Meet, so the Describe textarea labelled "What should this agent do? (system prompt)" was empty. The copied prompt only surfaced on the Launch summary card, with nowhere to edit it. "Draft it for me" had the same gap: the AI rewrite never came back into the textarea.

The trail only went backwards. Steps ahead of the current one were inert. From Meet, Powers / Summon / Launch looked like tabs and did nothing.

Rationale

This is not a regression. The Describe textarea has been the system prompt since the guided flow replaced the expert form (#635), and editing an existing agent still has its own Instructions field in the Define pane. The bug was that the copy and draft paths bypassed the one place the wizard lets you see and edit the prompt.

The trail change mirrors what #882 did for the Workspaces wizard.

Implementation

• The brief is the system prompt field end to end: agent-start with a source seeds it with the source's instructions; the PHP agent-draft action writes its rewrite back into it; the agent-step local carries it into instructions on any exit from Describe, so a trail jump does exactly what Continue does. The view's own seedFromBrief is gone in favour of the reducer.
• Every step except the current one is interactive. Launch's create still bounces to Meet when the name is missing.
• <os-step interactive> lifts its title (and an outlined chip) to full contrast on hover, through a new --os-ui-step-title-hover-color token falling back to the foreground colour. Filled chips have no border to recolour and keep their fill. The cursor stays the shell's arrow: the no-pointer policy in desktop.css is deliberate and left alone.
• The "No instructions yet" hint now points to Describe as well as Define.

Testing instructions

1. Enable Agents in OpenStation Preferences → Features and open WP Explorer → Agents with at least one agent.
2. Click Cast a new agent, then pick an existing agent under Start from someone. You land on Meet.
3. Click Describe in the trail: the textarea shows the copied prompt. Edit it, then click Launch in the trail. The summary card shows your edit.
4. Hover Powers, Summon or Launch from any other step: the label brightens, and clicking jumps there.
5. With an AI connector configured: type a brief, click Draft it for me, then go back to Describe. The textarea shows the drafted instructions.

Tests: the wizard locals and Describe render in apps/my-wordpress/parts/agents.test.ts, and the draft-action test in tests/phpunit/tests/myWordPressAppAgents.php.