docs: enhance guides with navigation, skill best practices, and README cleanup

[shinpr]

Summary

• Quickstart (EN/JA): Added "What's Next?" table linking common tasks to solutions, and library docs placement guidance with cross-references to use-cases and skills-editing-guide
• Use Cases (EN/JA): Added customization use cases — how to add library docs (with context-dependent decision framework) and team-specific rules as skills
• Skills Editing Guide (EN/JA): Expanded with naming conventions (gerund form), description best practices (third-person voice, activation rate data, good/bad examples from official docs, 1024-char constraint), and file reference depth guidance
• README (EN/JA): Updated Skills Editing Guide description, added library docs FAQ, consolidated slash commands table (frontend commands as footnote), replaced sub agents roster table with summary + link
• Overlap resolution: Quickstart now links to use-cases for detailed library docs decision table instead of duplicating it

Test plan

• Verify all cross-document links resolve correctly (quickstart → use-cases, quickstart → skills-editing-guide, use-cases → skills-editing-guide)
• Confirm README anchor links in table of contents still work after section name changes
• Review EN/JA content parity — same structure and information in both languages
• Check that no information was lost — detailed content moved to linked pages, not deleted

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