docs: restructure README to match gh CLI style — lean overview with manual links

[q-soriarty]

Description

Current README is too detailed — duplicates content from the manual (usage, config, architecture, full command table). Restructure to match gh CLI README: concise overview, quick install, link to manual for details.

Tasks

• Trim README to: badges, one-liner, short description, quick install (platform links), basic usage (3-4 commands), link to manual, contributing, related links
• Move detailed content to manual (already there)
• Add manual site link prominently
• Remove Architecture section (in CODEBASE_MAP.md)
• Remove Configuration section (in manual)
• Remove full Commands table (in manual)
• Keep Roadmap section (project-level, not user docs)

Reference

gh CLI README: https://github.com/cli/cli#readme

[q-soriarty]

Superseded by #132 (Jekyll documentation migration). Will be closed when #132 is merged.

[github-actions]

Closed by #136 (merged into develop).