feat: add OpenCode v2 plugin entrypoint

[peeyush-ciridae]

Summary

• add a native OpenCode v2 entrypoint at opencode-mem/v2
• adapt memory tools, prompt/context injection, internal generation, and lifecycle events to the v2 APIs
• filter the global v2 event stream by plugin location
• clear per-session injected context before each prompt to avoid stale context reuse
• preserve the existing v1 entrypoint and document v2 configuration

Validation

• npm run typecheck
• TypeScript build
• v1 and v2 loader contract tests: 8 tests passed
• package dry run confirmed dist/v2/plugin.js is published

Related to #173.