docs(ecosystem): add billion-context

[ranxianglei]

Adds billion-context to the Plugins table.

What it is: a transparent context-compression gateway for coding agents. OpenCode sessions route through it (base_url → local proxy) and history is folded into tiered summaries on the wire — the model sees a bounded window, the client keeps the full session.

Why month-long sessions: folding keeps the effective context small (100K is enough), so a single session can run for weeks without /clear — cumulative throughput in the billions of tokens.

OpenCode integration:

• npx bili plugin install opencode (one command, wires config), or
• add "billion-context" to the plugin array in opencode.json — the package exposes exports["./server"], so the bare name loads as a native plugin (server hooks), no extra package needed.

Repo: https://github.com/ranxianglei/billion-context

One-line diff, EN table only (per convention in previously merged ecosystem PRs).

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