docs(readme): add Full Tunnel mode section

[yyoyoian-pixel]

Summary

• Add a dedicated Full Tunnel mode section to the main README, between "How it works" and "Quick Start"
• Explains what Full Tunnel is, why you'd use it over apps_script mode (YouTube, UDP/Telegram, quota), the architecture, and what's needed to set it up
• Links to CodeFull.gs and the tunnel-node README for step-by-step setup

Previously Full Tunnel was only mentioned in passing in two FAQ answers — easy to miss for new users.

Test plan

• Verify rendered markdown on GitHub looks correct
• Confirm links to CodeFull.gs and tunnel-node/README.md resolve

🤖 Generated with Claude Code

[therealaleph]

@yyoyoian-pixel — the writeup is clear and accurate, but the README was just rewritten in v1.9.6 / v1.9.7 to be a short bilingual landing page (5-min Quick Start), with the full advanced reference moved to docs/guide.md and docs/guide.fa.md. Adding a 43-line Full Tunnel section to the short README would undo the redesign — Full Tunnel mode already has its own dedicated section in docs/guide.md#full-tunnel-mode.

What the short README currently says about Full Tunnel: it's mentioned twice in the FAQ ("YouTube videos don't play" and "Is it safe?" both link to the long guide's Full Tunnel section). That's the right depth for the landing page — a one-line pointer instead of a full explanation upfront.

If you'd like to improve the Full Tunnel section in docs/guide.md (where the detailed coverage actually belongs), I'd merge that gladly — your prose is good, and the diagram + "what you need" checklist would land well there.

Closing this PR; thanks for the contribution.

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_{[reply via Anthropic Claude | reviewed by @therealaleph]}