refactor(agents): remove GitHub Markdown line-break rule

[p-chan]

Why

The GitHub Markdown line-break rule was added in #550 after an agent hard-wrapped Japanese issue bodies mid-sentence, which GitHub renders as <br> in issues, PRs and comments. Current models no longer seem to do this, so the rule no longer needs to be loaded in every session.

What

• Remove the "Markdown 本文の改行" section from home/.agents/AGENTS.md

Notes

• Asked Codex and Claude, without the rule, to write the same issue body as the original failure; both wrote each paragraph on a single line. This was one sample each, printed to stdout rather than written via gh
• If mid-sentence line breaks reappear, restore the rule from feat(agents): add GitHub markdown line-break rule #550 (possibly as a personal-github reference)