feat(ecc): prune plugin 43→12 items, promote 7 rules to .claude/rules/ (#245)

ECC community plugin pruning: removed 530+ non-essential files
(.cursor/, .opencode/, docs/ja-JP, docs/zh-CN, docs/zh-TW,
language-specific skills/agents/rules). Retained 4 agents,
3 commands, 5 skills. Promoted 13 rule files (8 common + 5
typescript) to .claude/rules/ for CC native loading. Extracted
reusable patterns to EXTRACTED-PATTERNS.md.

.cursor/rules/golang-coding-style.md

@@ -1,32 +0,0 @@
----
-description: "Go coding style: gofmt mandatory, small interfaces, error wrapping with context"
-globs: ["**/*.go"]
-alwaysApply: false
----
-
-# Go Coding Style
-
-> This file extends [common/coding-style.md](../common/coding-style.md) with Go specific content.
-
-## Formatting
-
-- **gofmt** and **goimports** are mandatory — no style debates
-
-## Design Principles
-
-- Accept interfaces, return structs
-- Keep interfaces small (1-3 methods)
-
-## Error Handling
-
-Always wrap errors with context:
-
-```go
-if err != nil {
-    return fmt.Errorf("failed to create user: %w", err)
-}
-```
-
-## Reference
-
-See skill: `golang-patterns` for comprehensive Go idioms and patterns.