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/common-coding-style.md

@@ -1,53 +0,0 @@
----
-description: "Core coding style rules: immutability, file organization, error handling, input validation"
-alwaysApply: true
----
-
-# Coding Style
-
-## Immutability (CRITICAL)
-
-ALWAYS create new objects, NEVER mutate existing ones:
-
-```
-// Pseudocode
-WRONG:  modify(original, field, value) → changes original in-place
-CORRECT: update(original, field, value) → returns new copy with change
-```
-
-Rationale: Immutable data prevents hidden side effects, makes debugging easier, and enables safe concurrency.
-
-## File Organization
-
-MANY SMALL FILES > FEW LARGE FILES:
-- High cohesion, low coupling
-- 200-400 lines typical, 800 max
-- Extract utilities from large modules
-- Organize by feature/domain, not by type
-
-## Error Handling
-
-ALWAYS handle errors comprehensively:
-- Handle errors explicitly at every level
-- Provide user-friendly error messages in UI-facing code
-- Log detailed error context on the server side
-- Never silently swallow errors
-
-## Input Validation
-
-ALWAYS validate at system boundaries:
-- Validate all user input before processing
-- Use schema-based validation where available
-- Fail fast with clear error messages
-- Never trust external data (API responses, user input, file content)
-
-## Code Quality Checklist
-
-Before marking work complete:
-- [ ] Code is readable and well-named
-- [ ] Functions are small (<50 lines)
-- [ ] Files are focused (<800 lines)
-- [ ] No deep nesting (>4 levels)
-- [ ] Proper error handling
-- [ ] No hardcoded values (use constants or config)
-- [ ] No mutation (immutable patterns used)