docs: v2 plugins guide event.system example omits required type: "text" #48450

Summary

The V2 plugins guide's context-hook example pushes { text } onto event.system, but the
LLM.SystemPart schema enforced by SessionModelRequest.prepare requires type: "text".
Following the example verbatim makes every model call fail with Error: Schema validation failed.

Environment

• opencode version: 0.0.0-beta-19271
• OS: Linux 6.17.0-1022-azure (x86_64, Ubuntu)
• Terminal: TERM=xterm-256color (TERM_PROGRAM unset, COLORTERM unset)
• Shell: /usr/bin/zsh
• Install/channel: beta (@opencode-ai/cli)
• Active plugins: @plannotator/opencode@latest; one local plugin under ~/.config/opencode/plugins/

Reproduction

1. Create a V2 plugin with the context hook exactly as documented at
   https://opencode.ai/v2/docs/build/plugins ("Sessions" → "Model requests"):

   export default {
     id: "example",
     async setup(ctx) {
       await ctx.session.hook("context", (event) => {
         event.system.push({ text: "Keep the review focused on correctness." });
       });
     },
   };

2. Load it (e.g. ~/.config/opencode/plugins/example.js) and run any prompt:
   opencode2 run "hi".

Expected Behavior

The documented example works as written.

Actual Behavior

Every request fails before dispatch:

level=ERROR message="Failed to drain Session" cause="Error: Schema validation failed
    at SessionModelRequest.prepare (...)"

The Effect Schema issue pointer is ["system"][3]["type"] → MissingKey. LLM.SystemPart is
defined as:

{ type: "text"; text: string; cache?: LLM.CacheHint; metadata?: Record<string, unknown> }

so { text } alone is rejected.

Additional Context

• Correct form: event.system.push({ type: "text", text: "..." }) — adding type fixes it.
• Affected docs page: https://opencode.ai/v2/docs/build/plugins. Only the event.system example
  is wrong; the surrounding event.system.push(...) prose matches the bad example.
• Note: SessionContextHook's system: SystemPart[] type in the same page does not spell out the
  type: "text" discriminant either, so the omission is consistent in the docs.

"issues": [
  {
    "_tag": "Pointer",
    "path": ["system"],
    "issue": {
      "_tag": "Composite",
      "issues": [
        {
          "_tag": "Pointer",
          "path": [3],
          "issue": {
            "_tag": "Composite",
            "issues": [
              { "_tag": "Pointer", "path": ["type"], "issue": { "_tag": "MissingKey" } }
            ]
          }
        }
      ]
    }
  }
]