docs: add XP system overview

docs/XP_SYSTEM.md

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+# XP System
+
+PokeBuddy companions grow as developers work.
+
+XP can be awarded for local tasks, successful commands, tests, builds, lint, commits and future hooks.
+
+The first version keeps everything local in `~/.pokebuddy/state.json`.
+
+## Principles
+
+- Reward progress, not vanity metrics.
+- Prefer successful outcomes over raw lines of code.
+- Keep code and personal data local.
+- Make every reward feel delightful but not distracting.