docs: honor QWEN_HOME in common rule copy instructions

docs/QWEN-GUIDE.md

@@ -17,7 +17,7 @@ but reports eleven rule warnings: ten files in `rules/common/` and
 `rules/README.md` have no `paths:` frontmatter.
 
 This is intentional Qwen behavior: **extension rules must be conditional**.
-User rules under `~/.qwen/rules/` may instead be baseline instructions, loaded
+User rules under `~/.qwen/rules/` (or `$QWEN_HOME/rules/` when set) may instead be baseline instructions, loaded
 from the first request. See the [Qwen rules documentation](https://qwenlm.github.io/qwen-code-docs/en/users/features/rules/).
 Do not add artificial `paths: ["**/*"]` to shared ECC rules just to silence the
 warnings; that changes when they apply and does not preserve baseline semantics.
@@ -30,10 +30,10 @@ supplement the extension with a user-owned copy of **only** its common rules:
 ```bash
 # Inspect these policies first: they include TDD, coverage and delegation rules.
 # Stop if the destination exists; review/merge existing files instead of overwriting.
-test ! -e "$HOME/.qwen/rules/ecc/common" && \
-  test ! -L "$HOME/.qwen/rules/ecc/common" && \
-  mkdir -p "$HOME/.qwen/rules/ecc" && \
-  cp -R "$HOME/.qwen/extensions/ecc/rules/common" "$HOME/.qwen/rules/ecc/"
+test ! -e "${QWEN_HOME:-$HOME/.qwen}/rules/ecc/common" && \
+  test ! -L "${QWEN_HOME:-$HOME/.qwen}/rules/ecc/common" && \
+  mkdir -p "${QWEN_HOME:-$HOME/.qwen}/rules/ecc" && \
+  cp -R "$HOME/.qwen/extensions/ecc/rules/common" "${QWEN_HOME:-$HOME/.qwen}/rules/ecc/"
 ```
 
 For project-only scope, use `<project>/.qwen/rules/ecc/` as the destination;