Initial scaffold: open-memory plugin for OpenCode

- Plugin entry point with hooks: experimental.session.compacting,
  experimental.chat.system.transform, event
- Context tracker: SSE-based token tracking per session with
  green/yellow/red/critical thresholds
- Tools: memory_context, memory_compact, memory_summary,
  memory_sessions, memory_messages, memory_search, memory_plans
- History module: sqlite3 queries + markdown rendering
- Compaction: improved prompt emphasizing self-continuity
- Research docs: ARCHITECTURE.md + opencode-memory reference

opencode.json

@@ -0,0 +1,4 @@
+{
+  "plugin": ["@alkdev/open-memory"],
+  "$schema": "https://opencode.ai/config.json"
+}