docs: add AGENTS.md with the project's non-obvious constraints

Records what cost real time to discover: that rofi's -dump-theme normalises
values and cannot tell you what renders, that measuring screenshot pixels beats
reasoning about the box model, that GTK3's theme comes from gsettings rather
than settings.ini on Wayland, that ClearlyU shadows Private Use Area glyphs in
fontconfig fallback, and why accent extraction must not go through wal -i.

CLAUDE.md is a thin pointer at it, matching the user's global convention.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G6aeE37K4GHBsaM51yLTTq

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