docs: move QQ community section to top of README

[ranxianglei]

Follow-up to #371 — the Community/社区 section landed right before License at the bottom, easy to miss.

Moves it up to be the first content section in both READMEs:

• README.md: ## Community now after the header banner + host-support note, before ## Why?
• README.zh-CN.md: ## 社区 in the same position, before ## 为什么选择 billion-context

Content unchanged: one shared QQ group (1056132097) covering billion-context, billion-context-pi, opencode-acp. Docs-only; no version bump.

Fixes #372

[github-actions]

📦 Built Extension Artifact

Branch: 2026-09-11_readme-community-top (2453bb0)

Option A — Install from npm PR tag (recommended)

pi install npm:billion-context-pi@pr-373

Each push to this PR publishes a new version under the pr-373 npm tag.

Option B — Download artifact

1. Download the artifact from the Actions run
2. Extract the tarball and install:

tar xzf billion-context-pi-pr373.tgz
pi install ./package

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This comment is automatically updated on each push.