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.

contexts/dev.md

@@ -1,20 +0,0 @@
-# Development Context
-
-Mode: Active development
-Focus: Implementation, coding, building features
-
-## Behavior
-- Write code first, explain after
-- Prefer working solutions over perfect solutions
-- Run tests after changes
-- Keep commits atomic
-
-## Priorities
-1. Get it working
-2. Get it right
-3. Get it clean
-
-## Tools to favor
-- Edit, Write for code changes
-- Bash for running tests/builds
-- Grep, Glob for finding code