Add BILLION_CONTEXT_NATIVE stand-down guard (coexistence with bili native plugin) #438

Problem

opencode-acp has two bili-related guards:

1. Load-time: BILLION_CONTEXT_PROXY env → disabled
2. Config hook: /bili/ prefix in provider baseURLs → disabledByBiliProxy + deny own tools

Neither checks BILLION_CONTEXT_NATIVE. billion-context's host-native plugin (bili plugin install opencode, ships in billion-context v0.1.x, see billion-context#918/#919) sets BILLION_CONTEXT_NATIVE=opencode synchronously at module load and spawns its proxy asynchronously, rewriting baseURLs in its own config hook.

What happens today when both are installed (manual case)

• Load order opencode-acp first: its load-time check sees no BILLION_CONTEXT_PROXY (native's proxy isn't up yet) → stays armed. If its config hook also runs before the native plugin's rewrite, findBiliProxyProviders finds nothing → ACP stays armed for the whole process (guards are only re-evaluated on config reload).
• Both register tool.compress/decompress/search_context/acp_status — last registration wins; hooks (experimental.chat.messages.transform etc.) from opencode-acp still run with ITS ref space while tools route to billion-context's kernel → double machinery / mismatched refs.

billion-context's installer now strips opencode-acp entries from the global opencode.json (74701fb), but project-local installs and hand-edited configs can still produce the conflict.

Suggested fix

Mirror the pi extension (billion-context-pi) behavior: in the config hook (and ideally each tool/hook action), also stand down when process.env.BILLION_CONTEXT_NATIVE === "opencode":

// config hook, alongside findBiliProxyProviders
if (process.env.BILLION_CONTEXT_NATIVE === "opencode") disabledByBiliProxy = true

The env var is set synchronously at native plugin module load, so by the time any config hook or LLM request runs it is visible regardless of plugin load order.

Repro

1. opencode.json (global): "plugin": ["opencode-acp", "<configDir>/plugins/billion-context"] (the latter from bili plugin install opencode)
2. Start opencode 1.18.31 with a provider that has an explicit baseURL
3. Depending on hook order you get double-registered compress tools and/or both kernels armed

(Reported from billion-context PR #919 review — user spotted the coexistence hazard.)