codexbox v0.5.4 — a page per mode, and a corrected cron claim

Mode docs moved to docs/modes/ so each mode's env vars live in one place instead
of two. MCP mode was missing from the Table of Contents. CRON_MODE_HISTORY_DIR
was documented as relocating run history; it does not — the scheduler's history
root is fixed, and that variable only points the telegram bot at the cron inbox.
No image behaviour changed.