[FEATURE]: report plan usage (Go limits, Zen balance) to ACP clients

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Describe the enhancement you want to request

opencode acp already sends usage_update with the context size and the session's cost. What an ACP client still can't see is the account's plan standing:

• Go: the rolling 5-hour, weekly and monthly usage that GET /zen/go/v1/usage returns (#16017).
• Zen: the remaining balance (#10448).

Today an ACP client (Zed, HarnessDesk, others) has to find the user's key and call those endpoints itself, which it may not be able to do.

Suggestion: have the ACP server attach these figures itself. ACP has no standard plan-usage message yet, so _meta or an extension method is the sanctioned place. Either of these would work:

• a _meta block on usage_update:

{ "sessionUpdate": "usage_update", "used": 53000, "size": 200000,
  "_meta": { "opencode": { "go": { "rolling": { "percent": 19.5, "resetsAt": "…" }, "weekly": { … }, "monthly": { … } } } } }

• a custom request such as _opencode/usage, returning the same object.

In the meantime, HarnessDesk reads each session's cost from opencode.db (HarnessDesk/HarnessDesk#772), but that shows spend, not the plan's limits.

Start at the opencode acp handling for usage_update, then review the Go usage endpoint GET /zen/go/v1/usage and the Zen balance flow referenced in the issue. Compare the _meta and custom _opencode/usage options, and define completion as ACP clients receiving rolling, weekly, monthly, and balance information without querying account endpoints themselves.