[FEATURE]: Support enforced OTLP settings in managed configuration

Feature hasn't been suggested before.

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Describe the enhancement you want to request

OpenCode supports system-managed configuration and macOS managed preferences, but its OTLP exporter is configured only through inherited environment variables:

• OTEL_EXPORTER_OTLP_ENDPOINT
• OTEL_EXPORTER_OTLP_HEADERS
• OTEL_RESOURCE_ATTRIBUTES

This means an administrator cannot enforce an OTLP destination through /etc/opencode, %ProgramData%\opencode, or an ai.opencode.managed MDM profile. A user can unset or replace the environment variables when launching OpenCode.

Add dedicated managed OTLP settings:

{
  "telemetry": {
    "otlp": {
      "endpoint": "https://otel.example.com",
      "headers": "Authorization=Bearer token",
      "resourceAttributes": "deployment.environment.name=production"
    }
  },
  "experimental": {
    "openTelemetry": true
  }
}

Values present in the macOS ai.opencode.managed profile should have the highest precedence and should not be overridable by system-managed files, inherited environment variables, user configuration, or project configuration. For values omitted from the profile, the system-managed file should take precedence over inherited OTEL_* variables. Precedence should be evaluated per setting.

A dedicated OTLP block is preferable to a general managed env dictionary because it limits the capability to exporter settings OpenCode already supports.

Related: #12142, #25839