[feature] Prestige mode for Lvl 15 players

[EvanPaules]

Give top players something to chase once they hit Lvl 15 (10x Legend).

Idea

At level 15, claudexp prestige lets you reset total_xp (and level, session_count) in exchange for a permanent cosmetic star on your leaderboard username:

• 1st prestige: ⭐ evan
• 2nd: ⭐⭐ evan
• ...

Stars stack. You climb the ladder again each time. Prestige count shows up in claudexp stats and on the board.

Implementation sketch

• Add prestige_count integer column to profiles (new migration + update to cloud_schema.sql)
• New CLI command claudexp prestige — confirms, increments counter, zeros XP / level / session_count
• Leaderboard render prefixes username with ⭐ × prestige_count
• RLS: owner-token gate already covers it
• Consider adding a new achievement for first prestige

Open questions

• Require a confirmation phrase (type PRESTIGE) to avoid accidents?
• Cap the count? (probably fine to leave uncapped)
• Keep achievements or wipe them on prestige?