feat(games): add the generic d6 oracle game package

Serves as the reference implementation of the game package contract:
rules, spark tables, character backgrounds, and an example command.

games/generic/characters/README.md

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+# Character backgrounds
+
+The game's menu of character options: backgrounds, playbooks, and starting
+packages. Campaigns do not copy these files — the *actual* PC sheet lives in
+`campaigns/<name>/characters/`. This directory is where the options come from.
+
+## Files
+
+- `backgrounds.csv` — rollable starting backgrounds (`min,max,text,tags`).
+  Roll one with `journal background --campaign <name>`.
+
+Add more files here for playbooks, archetypes, or species. If a file should be
+rollable through `journal roll`, put it in `tables/` instead; files here are
+read by game commands or by the agent directly.