[codex] fix HUD context percentage stability

[franksde]

Summary

• derive HUD context percentage/bar from input tokens when token totals are available
• fall back to upstream used_percentage when token totals are missing
• document the input-side context behavior in both READMEs

Root cause

The HUD trusted context_window.used_percentage directly. During long tasks, upstream status-line data can reflect the latest response shape, so large or small recent output could make the displayed context percentage jump.

Validation

• npm test
• synced dist/agy-hud.js to the installed Antigravity plugin copy
• ran the installed plugin against testdata/statusline_payload.json