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/sessions/0001-the-mud-child.md

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+# Session 0001 — 泥巴孩子
+
+- **Campaign:** the-ballad-of-chalk
+- **Game:** cloud-empress
+- **Date:** 2026-09-13
+- **PC:** 弗莱格（酸的）
+
+## Scene 1 — 盐脊道上
+
+`spark roll 20 → "一个浑身涂满泥巴的孩子，说自己是村庄唯一的幸存者。" [npc, child]`
+`weather roll 7 → "滑界涟漪——周围的现实被扭曲。" [slip]`
+
+**Scene:** 低地边缘，盐碱荒原。天是滑界涟漪，云层像被谁用手指抹开，现实的边缘在弗莱格
+眼角余光里微微走样。风里全是盐味。他一个人走在干涸的盐脊道上，行囊里那根白垩又被
+拇指摩过一遍。就在前面路当中，站着一个浑身涂满湿泥的孩子，一动不动地看着他。

campaigns/the-ballad-of-chalk/sessions/README.md

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+# Sessions
+
+One file per session, named `NNNN-slug.md` (zero-padded, e.g. `0001-the-ash-gate.md`).
+Session logs are append-only; see `AGENTS.md` for the format.