README/docs missing zero upgrade, /new, and community link

[PierrunoYT]

Summary

A few recently merged features and a community resource aren't reflected in the docs:

• /new (added in polish: cut per-turn overhead, stabilize cache, add /new #478) — not listed in the README's slash-command table.
• zero upgrade / zero update --apply (added in feat(update): add zero upgrade command to apply self-updates #461) — not listed in the README's command list, and docs/UPDATE.md only documents --check, not the install/apply flow.
• The project's Discord server isn't linked anywhere in the README.

Proposed fix

• Add /new to the slash-command table in README.md.
• Add zero upgrade to the Common Commands list in README.md.
• Expand docs/UPDATE.md to document --apply / zero upgrade behavior (npm delegation, standalone binary replace, Windows rename-aside, --target restriction).
• Add a Discord badge + Community section linking https://discord.com/invite/N9CrWCH7b in README.md and README_ZH.md.

I'll open a PR shortly with these changes.

[PierrunoYT]

Heads up: the Discord invite added in #489 (`N9CrWCH7b`) expires 2026-08-02 (~1 month out). It does resolve to the correct "Gitlawb Community" guild, but a headline-README link that stops working in a month isn't great.

Someone with Manage Server / Create Invite rights on that Discord server needs to generate a never-expiring invite to replace it. I don't have Discord access to do this myself. Once a permanent invite link is available, I'll swap it into README.md and README_ZH.md on the PR.