Shorten README and lead with agent-assisted setup

[alexanderqchen]

Summary

The README currently mixes basic setup with detailed operational and development instructions. This draft makes the first action a copy-and-paste prompt for a Codex agent, followed by a short overview and a plain-language settings table.

• Reduces the README from 1,121 to 447 words (60% shorter).
• Keeps the overnight defaults, active-task behavior, permission requirement, sleep limitation, and English-dialog scope visible.
• Moves manual installation, CLI examples, migration notes, troubleshooting, logs, uninstall steps, and implementation/development details into docs/reference.md.
• Adds background Accessibility verification guidance to the reference so agent-assisted setup does not mistake inherited foreground access for a working service permission.

Validation

• Relative documentation links, logo path, Markdown fences, and whitespace checked.
• zsh -n bin/codex-update-helper passed.
• ./tests/test.sh passed all 20 test groups.

Documentation only. Ready for editorial review as a draft PR; no runtime behavior changes.

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