feat: Better empty-state UI when no sessions exist

Problem

When a user first runs mi6 with no sessions (or with frameworks not enabled), they see an empty table with no clear guidance on what to do next.

The current onboarding message appears in the error panel area, which is easy to miss and feels like something went wrong rather than a helpful prompt.

Current Experience

User sees empty table and thinks: "Is it broken? Did I do something wrong?"

Proposed Experience

When the session list is empty AND frameworks are not enabled, show a centered helpful message:

              No sessions yet

     Run `mi6 enable` to start monitoring:
       - Claude Code (detected)
       - Gemini CLI (detected)

     Press q to quit, ? for help

When frameworks ARE enabled but no sessions yet:

              Waiting for sessions...

     mi6 is monitoring: Claude Code, Gemini CLI
     Start a coding session to see it here.

Scope

• Only affects empty state rendering in TUI
• Reuse existing framework detection logic from check_onboarding_status()
• No new commands or flags needed