docs: fix cp -r flatten install instructions that silently overwrite rules

The configure-ecc skill (English source) and ja-JP top-level README plus
its configure-ecc translation documented `cp -r .../<lang>/* <target>/rules/`
for installing rule directories. rules/README.md explicitly warns against
this form because:

- Common and language-specific directories contain same-named files
  (coding-style.md, testing.md, patterns.md, hooks.md, security.md).
  Flattening makes each language overwrite the previous and common.
- The relative `../common/<file>.md` references in language rule files
  break when common/ is no longer a sibling directory.

This is silent — the user gets only the last language's rules with no
error message.

Replace with the directory-form copy already documented in
rules/README.md:

    cp -r .../<lang> <target>/rules/<lang>

Scope: English source + ja-JP. Closes #1879. Other locale translations
(zh-CN, ko-KR, pt-BR, tr) carry the same pattern and can follow up as
separate PRs to keep this change reviewable.

skills/configure-ecc/SKILL.md

@@ -234,13 +234,13 @@ Options:
 
 Execute installation:
 ```bash
-# Common rules (flat copy into rules/)
-cp -r $ECC_ROOT/rules/common/* $TARGET/rules/
+# Common rules
+cp -r $ECC_ROOT/rules/common $TARGET/rules/common
 
-# Language-specific rules (flat copy into rules/)
-cp -r $ECC_ROOT/rules/typescript/* $TARGET/rules/   # if selected
-cp -r $ECC_ROOT/rules/python/* $TARGET/rules/        # if selected
-cp -r $ECC_ROOT/rules/golang/* $TARGET/rules/        # if selected
+# Language-specific rules (preserve per-language directories)
+cp -r $ECC_ROOT/rules/typescript $TARGET/rules/typescript   # if selected
+cp -r $ECC_ROOT/rules/python $TARGET/rules/python            # if selected
+cp -r $ECC_ROOT/rules/golang $TARGET/rules/golang            # if selected
 ```
 
 **Important**: If the user selects any language-specific rules but NOT common rules, warn them: