Add support for local executable override (OPENCODE_EXECUTABLE) #11283

Problem

Currently, Zed's OpenCode extension does not support using a local dev version of OpenCode. Unlike other agents (Claude Code, Gemini CLI), there's no way to override the executable path.

Current State

• Claude Code: Supports CLAUDE_CODE_EXECUTABLE environment variable
• Gemini CLI: Supports ignore_system_version setting
• OpenCode: ❌ No override mechanism available

This makes it difficult for developers who want to:

1. Test local changes to OpenCode
2. Use a custom-built version
3. Develop and debug OpenCode features

Expected Solution

Add support for an environment variable like OPENCODE_EXECUTABLE to override the binary path, similar to Claude Code:

{
  "agent_servers": {
    "opencode": {
      "env": {
        "OPENCODE_EXECUTABLE": "/path/to/local/opencode"
      }
    }
  }
}

This would allow developers to point Zed's OpenCode extension to a local dev build instead of downloading from GitHub releases.

Impact

This is a common pattern for agent development:

• Claude Code has it ✅
• Gemini CLI has it ✅
• OpenCode needs it ❌

Without this, developers must use the custom agent approach (no icon, no proper integration) or constantly publish to GitHub releases for testing.