feat: add the-ballad-of-chalk campaign

Cloud Empress campaign for 弗莱格, a sellsword obsessed with chalk.
Ships the brief, PC sheet, state files, a starting location, and the
session 0001 log. The opening hook and hidden truth live in gm/.

campaigns/the-ballad-of-chalk/house-rules.md

@@ -0,0 +1,5 @@
+# House rules
+
+This campaign's overrides and additions to the game's rules. The game package
+is authoritative; anything written here takes precedence for this campaign
+only. Leave it empty if the game runs as written.