docs: restructure README (EN/JA) for readability

[reoring]

Summary

• Add concise Features at a Glance / 特徴 section near the top so readers can quickly grasp what Roboppi does
• Move the detailed capabilities list into a Reference section at the end (no content deleted)
• Reorder sections for natural reading flow: intro → features → install → quickstart → architecture → reference details
• Demote Workflow YAML, Agent Catalogs, Branch Safety, IPC Transport sections as subsections under Reference
• Add hero image to Japanese README for consistency with English version

Before / After structure

Before	After
Intro	Intro (condensed to one paragraph)
What Roboppi Can Do (massive list)	Features at a Glance (8 one-liners)
Architecture	Install
Install	Quickstart
Quickstart	Architecture
Workflow YAML	---
Agent Catalogs	Reference (Capabilities in Detail, Workflow YAML, Agent Catalogs, Branch Safety, IPC)
Branch Safety	Documentation
Supervised IPC	Development
Documentation	Status
Development
Status

Test plan

• Verify all original content is preserved (no technical information removed)
• Check that internal links and doc references still work
• Review both EN and JA versions render correctly on GitHub

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