docs: add Japanese harness investigation report - #1

Motivation

• Provide a concise, Japanese-language investigation of OpenCode's assistant/agent harness focusing on its runtime flow and the prompts that drive behavior so readers can quickly understand and navigate the code.

Description

• Add docs/harness-investigation-ja.md documenting the harness architecture (SessionPrompt, Agent, SystemPrompt/InstructionPrompt, ToolRegistry/PermissionNext, LLM.stream, SessionProcessor), prompt composition (provider/system/agent/plan/tool prompts), control points, and a recommended code-reading order.

Testing

• Ran repository checks git status --short and git rev-parse --abbrev-ref HEAD and created a commit containing the new file, and no functional tests were required since this is a documentation-only change.

Codex Task