docs: repository README, contributor rules, scenario template and authoring guide

README leads with the finding that motivated the repository -- pi emits the
skills section only when a tool named 'read' is active, so Curator's
--no-tools --skill combination made its policy unreachable -- with the measured
before/after table and instructions to reproduce it at zero token cost.

AGENTS.md sets seven rules for anyone changing this repository. The third is the
one that matters most: verify pi's behaviour with a probe rather than inferring it
from the docs. Three claims in the first draft of these documents were wrong and
were only corrected by running one.

The _template scenario carries the isolation defaults and inline warnings at the
places where mistakes have already cost time: --no-tools disabling the skills
mechanism, cwd anchoring .pi discovery, the read override being mandatory rather
than optional, and allowed-tools frontmatter not being enforced in 0.84.3.

scenarios/_template/workspace/.pi/APPEND_SYSTEM.md

@@ -0,0 +1,19 @@
+# <scenario> 长期职责
+
+> 放在 `.pi/APPEND_SYSTEM.md` 而非 `AGENTS.md`：context file 会从每一级父目录
+> 加载，`AGENTS.override.md` 只屏蔽同目录（已实测），唯一有效开关是 `-nc`，
+> 而它会连本目录的 context file 一起关掉。系统提示文件不受 `-nc` 影响。
+>
+> 身份与输出格式在 `.pi/SYSTEM.md`；本文只写会随时间演进的领域职责。
+
+## 职责
+
+<会演进的领域职责与判断标准。>
+
+## 默认策略
+
+<领域默认值，例如优先级、回退顺序、什么算完成。>
+
+## 已知能力边界
+
+<当前做不到的事，以及应当如何坦率说明。>