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.

.opencode/commands/checkpoint.md

@@ -1,67 +0,0 @@
----
-description: Save verification state and progress checkpoint
-agent: build
----
-
-# Checkpoint Command
-
-Save current verification state and create progress checkpoint: $ARGUMENTS
-
-## Your Task
-
-Create a snapshot of current progress including:
-
-1. **Tests status** - Which tests pass/fail
-2. **Coverage** - Current coverage metrics
-3. **Build status** - Build succeeds or errors
-4. **Code changes** - Summary of modifications
-5. **Next steps** - What remains to be done
-
-## Checkpoint Format
-
-### Checkpoint: [Timestamp]
-
-**Tests**
-- Total: X
-- Passing: Y
-- Failing: Z
-- Coverage: XX%
-
-**Build**
-- Status: ✅ Passing / ❌ Failing
-- Errors: [if any]
-
-**Changes Since Last Checkpoint**
-```
-git diff --stat [last-checkpoint-commit]
-```
-
-**Completed Tasks**
-- [x] Task 1
-- [x] Task 2
-- [ ] Task 3 (in progress)
-
-**Blocking Issues**
-- [Issue description]
-
-**Next Steps**
-1. Step 1
-2. Step 2
-
-## Usage with Verification Loop
-
-Checkpoints integrate with the verification loop:
-
-```
-/plan → implement → /checkpoint → /verify → /checkpoint → implement → ...
-```
-
-Use checkpoints to:
-- Save state before risky changes
-- Track progress through phases
-- Enable rollback if needed
-- Document verification points
-
----
-
-**TIP**: Create checkpoints at natural breakpoints: after each phase, before major refactoring, after fixing critical bugs.