Sessions started with slash commands have 'No prompt' and are unsearchable in /resume

Problem

Sessions that begin with a slash command (e.g., /goal-planning admin chat) are indexed with firstPrompt: "No prompt", making them unsearchable via /resume.

Steps to Reproduce

1. Start a new session with a slash command: /goal-planning admin chat
2. Have a long session with substantial content
3. Try to find the session later using /resume and search for keywords like "goal", "admin", "chat"
4. Session doesn't appear in results

Root Cause

The first user message in these sessions is the command metadata:

<command-message>goal-planning</command-message>
<command-name>/goal-planning</command-name>
<command-args>admin chat</command-args>

The session indexer sees this XML structure and sets firstPrompt: "No prompt" instead of extracting searchable text.

Session Index Entry

{
  "sessionId": "b9a42430-9663-470c-a0cf-6565fc641d55",
  "firstPrompt": "No prompt",
  "messageCount": 9,
  ...
}

Expected Behavior

Sessions started with commands should still be searchable. Options:

1. Extract command name and args as searchable text (e.g., firstPrompt: "goal-planning admin chat")
2. Use the first substantive user message after the command expansion
3. Index the expanded skill/command content

Impact

• Large, important sessions become effectively lost
• Users must remember to set custom names immediately, and remember what they named them
• Workaround requires manually searching session file contents with grep